An ERP billing system looks simple from the outside — create an invoice, take a payment. In practice it's one of the most detail-heavy things you can build, because every edge case is someone's real money. Here's what shipping one taught me.

Get the Data Model Right First

Everything in an ERP hangs off the data model. Customers, products, taxes, invoices, line items, payments — and the relationships between them — need to be right before you write a single screen. A weak schema here turns into months of patching later. I spent real time modeling this before touching the UI, and it paid off every week after.

The Edge Cases Are the Product

Partial payments, refunds, discounts, tax rounding, currency formatting, cancelled invoices, edits after issue — these aren't "extras," they're the actual job. Users hit them constantly, and getting rounding or tax wrong erodes trust instantly. Treat edge cases as first-class features, not afterthoughts.

• Never store money as floats — use integers or decimals
• Make invoices immutable; correct with credit notes
• Keep an audit trail of every change

Performance Under Load

Reports and dashboards are where ERPs slow down. Aggregations over thousands of invoices need proper indexing, and sometimes pre-computed summary tables. I learned to profile the real queries against real data volumes early, instead of waiting for a client to feel the lag.

Keep It Maintainable

A business app lives for years and grows constantly. Clear module boundaries, a service layer separated from controllers, and consistent naming meant new features slotted in without breaking old ones. Boring, disciplined structure is what keeps a big app pleasant to work on.

Wrapping Up

Building an ERP is a masterclass in caring about details. Respect the data model, embrace the edge cases, and structure for the long haul. Planning a business platform? I'd love to help.