Integrating CRM and ERP with REST APIs means a customer record, an order or a payment status exists once and arrives where it is needed, without copy-paste. It is not a magic “connection”. It is a contract between two systems: which data moves, in which direction, and what happens when one side is down or inconsistent.

Most SMEs start with a CRM for sales and an ERP (or spreadsheets plus invoicing) for the rest. The two worlds talk by voice, email or export. That works at low volume. Then you get duplicate accounts, quotes that never land in the ERP, invoices detached from the deal, and nobody knows which list is true.

What to align first (and what to leave)

Do not integrate “everything”. Integrate the minimum that removes repeated work and visible errors.

  • Customer records. A shared key (VAT number, internal code) matters more than the display name. Without it every sync creates duplicates.
  • Opportunities / orders. When a deal becomes an order, the ERP must know without someone retyping it. The CRM should not become the warehouse.
  • Statuses. “Won”, “invoiced”, “paid”, “shipped” need a map. If the vocabularies do not match, the API ships confusion faster than email.
  • Products and price lists. Only if sales actually sell from a real catalogue. Otherwise the CRM can stay descriptive and the ERP remains the price owner.

Call notes, email, deal attachments can usually stay in the CRM. Inventory, accounting, due dates stay in the ERP.

REST is the transport, not the strategy

A REST API exposes resources (customers, orders) with clear operations and authentication. You need documentation, versioning and rate limits. If the ERP has no API, you may go through middleware, a read-only database or files — knowing that is more fragile.

Decide direction: CRM is master of leads and ERP is master of invoices, or one system is the source for each object. Dual master is the most common reason projects never end.

Realtime sync is worth it for stock or payment status. For accounts, a job every few minutes is often enough and easier to debug. Webhooks (the system notifies on change) cut useless polling, but the other side must be reachable and you must handle retries.

Mistakes that explode time and cost

Mapping fields “in case we need them”. Ignoring duplicates you already have. Not defining who fixes a conflict. No test environment with fake data. No request logs. Asking the integration to “fix the process too”: if sales never close deals and finance never uses statuses, the API replicates the mess.

A serious integration includes: an inventory of systems and fields, master-data rules, error handling and replay, tests on real cases (same customer with two sites, partially fulfilled order, credit note).

How to start small

Pick one flow: for example “won customer in CRM → account and order in ERP”. Measure it (how many manual copies a week). Only then add invoices back to the CRM or status updates. Same idea as an MVP: visible value before a universal platform.

DPH designs API integrations between CRM, ERP and existing software. If you want to know whether your case is a focused connector or a wider programme, request a quote with the system names and the flow you currently do by hand.