The account record is clean — tax ID attached, addresses separated, system of record agreed. Now the first MIS-generated invoice is sitting in the queue, ready to cross into NetSuite, Business Central, Xero, or QuickBooks. This is where a different set of problems starts.
None of what follows will throw a loud error. Most of it will post silently and look completely normal until someone reconciles the books, an auditor asks a question, or a customer disputes a charge. The cost is measured in correction journals, credit notes, and days added to your DSO.
1. Map every tax code before the first invoice, not after.
Do not assume that "non-taxable" in the MIS means "non-taxable" in the ERP. ERPs are built to recalculate tax on import rather than trust the value an external system sends. A product marked exempt in the MIS may require a specific item tax group or a linked exemption certificate on the ERP side — and without that mapping, the ERP quietly recalculates it as taxable with no error message, producing a tax total higher than what the customer was billed while both systems report a successful sync. Walk every tax code pairing before go-live, not when the first mismatch surfaces on a VAT return.
2. Know whether your rounding happens at the line or at the document.
A one- or two-cent gap on a multi-line invoice is almost never a sync failure. It is the signature of a rounding-convention difference: one system rounds each line and sums the results; the other sums first and rounds once. Both calculations are arithmetically correct. What matters is that your ERP's flat-dollar tolerance threshold — commonly set at ±$10 — is wide enough to absorb these legitimate penny differences, which also means it is wide enough to silently absorb a systematic mapping error that falls below the same threshold. Know what your tolerance is set to, and know what it could be hiding.
3. Agree on which date sets the exchange rate — and design the integration to expect a variance.
The MIS typically locks the foreign-exchange rate at job creation or quoting. The ERP applies its own rate at PO creation or invoice posting, depending on configuration. Neither system is wrong. But the two rates will almost always produce a different functional-currency amount, and if the integration is not designed to route that difference to an FX gain/loss account, it will flag the mismatch as a sync error on every foreign-currency invoice you send. Settle this question with your implementation team before the first international invoice posts.
4. Coordinate invoice number sequences, and document every gap.
The ERP may impose its own numbering sequence on import, silently breaking the audit trail back to the MIS job number. Freight carriers compound this: a carrier will reissue an invoice with a suffix — INV-1234 becomes INV-1234-A — to add a missed charge, and standard AP software treats the suffix as a new, unique invoice and pays it even when the original is already settled. Before go-live, decide which number is the system of record, cross-reference it in both systems, and build a log of any gap in sequence. Auditors ask about gaps; "the ERP renumbered it" is an answer that requires documentation to be credible.
5. Check the posting period before you transmit, not after.
An invoice dated on the last day of a month in the MIS may arrive in the ERP hours or days later — after that period has been closed. SAP explicitly documents this as a blocking error; other ERPs handle it differently, some failing silently, some force-posting to the open period without telling anyone. An invoice that lands in the wrong accounting period distorts your revenue recognition and, if it carries a shipment date, your late-delivery paperwork. Make closing the MIS period and closing the ERP period a coordinated step, not two independent ones.
6. Give freight, pallets, plates, and overrun charges their own line mapping — not a catch-all code.
These charges are the ones most likely to have no part number in the MIS, and they are the ones most likely to misclassify silently in the ERP. Freight invoices alone carry a meaningful rate of discrepancy — linehaul, fuel surcharges, detention, and accessorial charges frequently don't resolve cleanly to a single GL account. An unapproved expedited-shipping charge, for example, falls outside standard percentage-tolerance rules because there is no PO line to compare it against. Map each charge type to a specific account before the first invoice runs. A catch-all code looks fine until you need to read a cost-centre report or defend a margin.
One or two days off DSO — recovered by billing accurately on the first attempt rather than chasing a correction — is worth more in working capital than most process investments a mid-size converter makes in a year. These six checks exist to make the first invoice the right invoice.
If you are in the middle of a first integration cycle and any of these questions don't have a clear answer yet, we are glad to talk it through — no agenda, just the specifics of how your shop floor and your ERP model the same transaction.