Thanks for the thorough details Min Liu.
First, the bug: good catch — we were sending the wrong address to carrier services. The fix is in progress.
On how rates get chosen: this behavior is still rolling out, so you may not see it immediately, but here’s how it will work:
Across multiple shipping options
- If more than one option is needed to ship the cart, their rates sum. Applies to every option type (carrier, flat, weight-based, order-amount-based).
- Example: “Standard” ships item A, “Oversized” ships item B → buyer pays Standard + Oversized
Within a single option
- Flat or order-total-based → the highest applicable rate is used.
- Carrier or weight-based → summed. E.g. a carrier returns a rate per package for 2 packages → the two are added.