When you need this

A lead arrives on the site and someone retypes it into the CRM. Stock lives in the accounting system and is copied to the site every morning by hand. Two systems both claim to know the price. Each is paid for daily in someone’s time and in errors that surface a week later.

What I actually build

One direction or both, decided on purpose. Both ways costs more: it needs a rule for when the same record changed in both places, and that rule is yours, not mine.
Nothing lost when the other side is down. Every outside system fails sometimes; when it does, the lead waits and is sent again. You do not hear about it from the customer.
A record you can read. When someone says “the lead never arrived”, the answer is a line showing what was sent, when, and what came back — not a shrug.
Your keys to the other system kept out of the code and out of the database, so a leaked backup does not leak your CRM.
The other system’s limits respected, so a bulk import does not get your account suspended on the worst day.

Where this shows up in my work

A course catalogue that has to work in three languages: enquiries go straight into the Telegram channel the sales team already has open, instead of an inbox checked at the end of the day. A two-way link with a CRM is not on a published case yet.

What you get

Data entered once, a record of every hand-over, and a written page saying what goes where — the document your next developer will ask for. The price depends on the other side: a system with documentation and a test account is a short job; one that has to be worked out by trial is a different project. I tell you which before the price is fixed in writing, not after.