A client asked us to "just add one more carrier" at checkout. Two days, we said. Then we found out each carrier rounds weight and dimensions its own way, so the quoted rate and the real label cost drifted apart: pennies per order, refunds and support tickets by month end. Now a carrier is its own contract with its own rounding tests, not just a connector.
You have probably underestimated an integration yourself, somewhere, because it had an API.