AP automation connects to an ERP through several patterns, including a native API, a vendor-managed connector, third-party middleware or iPaaS, or flat file transfer. This guide compares the three broad patterns most often used in vendor evaluations: native API, third-party middleware, and flat file. The method determines how much of your ERP structure the AP platform can see, when coding errors surface, and who ends up fixing them. Native connections read and write ERP records directly. Third-party middleware routes data through a separate translation platform. Flat file transfers move batches of records as files on a schedule. A vendor-managed connector, such as Stampli Bridge, is a different model from third-party middleware and should be evaluated on its own ownership and support terms.
Here is how each method works, what each one costs you in practice, and how to confirm which one a vendor is actually selling.
The three ERP integration methods
Most AP platforms on your shortlist use one or more of these methods, depending on the ERP. Some also use a vendor-managed connector, which is distinct from third-party middleware. That is why the question has to be asked per ERP rather than per vendor.
| Method | How it connects | Sync timing | Where errors surface | Typical fit |
|---|---|---|---|---|
| Native API | Calls the ERP’s own published API directly, with no system in between | Real time or every few minutes | Inside the AP platform, before the record posts | ERPs with a documented API and a partner certification program |
| Middleware/iPaaS | Routes data through a separate, third-party translation platform that maps fields between systems | Scheduled or event-driven, with a queue | In the middleware log, after the AP platform releases the record | Environments with heavy customization, or ERPs without a usable write API |
| Flat file | Writes batch files that the ERP imports on a schedule | Batch, typically hourly to daily | In the ERP import log, after the batch runs | Legacy, on-premise, or locked-down ERPs |
One clarification worth making early, because it gets misused in sales conversations: no method is universally better, and a vendor-managed connector is not automatically middleware. In this article, “middleware” means a separate third-party translation platform that sits between the AP platform and the ERP. Flat file is the correct answer for some legacy systems, and third-party middleware is the correct answer for environments where a native write path does not exist. What matters is whether the method matches the ERP you actually run, who owns the connection, and whether the vendor is straight with you about which one you are getting.
How native API integration works
A native API integration calls the ERP’s own interface directly. The AP platform reads your chart of accounts, entities, dimensions, vendors, and open purchase orders from the source, and writes invoices and payments back the same way. There is no translation layer and no intermediate vendor.
Most modern ERPs publish this path and run a certification program around it. Oracle NetSuite operates the Built for NetSuite program, and NetSuite’s own documentation notes that the badge must be renewed for every major release and can be revoked if a SuiteApp stops meeting program requirements. Acumatica, Microsoft Dynamics 365, and Oracle Fusion all publish REST or OData interfaces that support direct record-level reads and writes.
The practical advantage is timing. Because the platform can query the ERP while a person is still working the invoice, it can validate coding against live ERP rules before anything posts. That is the difference that shows up at month-end, and it is worth understanding how ERP integration works alongside AP automation before you compare feature lists.
Native is not automatically deep, though. A vendor can hold a valid certification and still support only header-level coding. Certification confirms the connection meets platform standards. It does not confirm field coverage.
How middleware and iPaaS integration works
Third-party middleware sits between the AP platform and the ERP, receiving data from one side, mapping it to the other side’s structure, and passing it along. Oracle Integration Cloud is a common example in Fusion environments, and general purpose integration platforms fill the same role elsewhere. This is different from a vendor-managed connector or on-premises bridge that the AP provider owns and supports as part of its ERP integration.
This model solves real problems. It handles unusual customizations, connects systems that were never designed to talk to each other, and gives IT a single place to manage many integrations at once.
It also introduces three costs that AP teams feel directly. The first is a delay: records move on the middleware’s schedule, not yours. The second is error ownership, since a rejected invoice surfaces in a middleware log that your AP team probably cannot see. The third is maintenance, because new dimensions, entities, or custom fields require someone to update the mapping. Teams running AP across separate companies and multiple ERP systems tend to hit that maintenance cost first, since every new entity is a new mapping to keep current.
A useful question here: who owns the middleware license, and who fixes it when a mapping breaks? If the answer is a third party, that is a real dependency worth pricing. Do not apply this question or the third-party middleware label to a vendor-managed connector without first confirming how that connector is owned, updated, and supported.
How flat file integration works
Flat file integration moves records as batch files. The AP platform generates a file, the ERP imports it on a schedule, and validation happens during that import.
Oracle documents this pattern clearly. Its File-Based Data Import process for Payables has you populate a spreadsheet template, generate CSVs, load them through the Load Interface File for Import scheduled process, and then run Import Payables Invoices, which creates invoices from records staged in the Payables open interface tables. You then search for and validate those invoices inside Payables. Oracle also documents the boundaries: the FBDI spreadsheet does not support creating prepayment invoices, and it accepts URL attachments only rather than attached documents.
Microsoft publishes a comparable file-based path for Dynamics 365 Finance and Operations through the Data Management Framework and its recurring integrations API, both of which run as asynchronous batch jobs rather than live calls.
Neither is a flaw. Both are supported, documented, and appropriate for bulk work. The tradeoff is simply that validation happens after your team has finished working the invoice, so exceptions come back as a queue rather than a prompt.
What the method changes in daily AP work
The connection method decides four things your team lives with every month.
- Coding depth. Whether the platform can present the dimension values your ERP considers valid for a given account, entity, or project, and whether it can code at line level rather than header level only.
- Error timing. Whether a bad tax code or missing dimension is caught while the invoice is open, or discovered days later in a log.
- Error ownership. Whether AP fixes its own exceptions, or files a ticket with IT or a systems integrator.
- Change cost. What happens when you add an entity, a cost code structure, or a custom field. Native paths often pick these up on the next sync. Mapped paths usually require a change request.
That last point compounds. A structure that works at three entities can require meaningful rework at fifteen, which is why the question belongs in the evaluation rather than the implementation.
How to verify which method a vendor uses
Vendor sites tend to say “integrates with” and stop there. Six questions get you past it.
- Which method connects to my specific ERP? Ask per ERP, not per product. Many vendors are native on one system and file-based on another.
- Is there a marketplace listing or certification, and what is its current status? Then check the listing yourself rather than taking the claim.
- How often does data move, and in which direction? “Real time” and “every four hours” are both defensible answers, but only one of them is real time.
- Where do rejected records appear, and who has access to that log? If AP cannot see it, AP cannot own it.
- What happens when we add an entity or a custom field? Automatic on next sync, or a scoped change request?
- What is not documented? A vendor who says a behavior is not documented is being more useful than one who improvises an answer. Treat an absence of documentation as an open question to test in the sandbox, not as a yes or a no.
Run those against the criteria for choosing the best AP integration for your ERP and you will have a defensible comparison rather than a feature checklist.
Where Stampli fits
Stampli connects to leading ERPs through native API, Bridge, and file-based integrations, with the ERP remaining the system of record throughout. Stampli Bridge is a lightweight, Stampli-owned on-premises connector that facilitates communication with the ERP. It is not third-party middleware: there is no separate third-party license owner, and the connector is maintained and auto-updated by Stampli without requiring ongoing IT involvement. On native API and Bridge paths, Stampli mirrors the chart of accounts, entities, dimensions, vendors, and approval hierarchies, and validates transactions against the ERP’s own rules before posting. Stampli AI performs on average 87% of finance work across 2,700+ unique fields, and every suggestion is reviewed by a person before it posts. You can see how that plays out on a single system in our walkthrough of the mechanics behind the NetSuite integration, or browse the full list of supported accounting systems and ERPs.
Integration method is the first thing to settle in an AP evaluation and the last thing most teams look at. Sort it early, per ERP, in writing. If you want to see what a native, validated connection looks like against your own chart of accounts, request a demo and bring your real coding structure.
