A shipping rate shown at checkout is not a price.
It is a quote with an expiry, and the carrier is under no obligation to honour it when you buy the label two hours later.
This is one of the quietest money leaks we see in cross-border e-commerce. Rates come from one API call at checkout. Labels come from a different call later, after payment, often after the warehouse has already packed and weighed the parcel.
Between those two moments things move. Rate tables get updated. Surcharges change. The real weight and dimensions are not the ones the customer's basket estimated. The address gets normalized into a different zone than the one you quoted.
If you treat the checkout number as a fact, the difference lands quietly on your margin, and nobody notices until someone reconciles a month of shipments.
What works better for us: store the quote as a record, not a number. Carrier, service level, the exact inputs you quoted on, the carrier's quote id when it gives one, and a validity window. At label purchase, re-quote and compare against that record.
Now the delta is visible, and it becomes a product decision instead of an accident: absorb it below a threshold, re-price above it, refuse and escalate when the service is no longer available at all.
The shift that matters is treating that gap as expected rather than exceptional. Most multi-carrier bugs we have debugged were never API failures. They were one number from checkout being reused as truth three steps downstream.
If you run multi-carrier shipping, you probably already have a line drawn somewhere between absorb and re-price, and someone who owns that number, ops or engineering.