Buy the label where you are already standing

A carrier rate is a price somebody is about to pay, and it expires. Re-quoting silently under the button would make the number on screen and the number charged two different things.

Changing the box clears the rates for the same reason. Carriers bill on dimensional weight, so a different box is a different price, and old rates sitting beside new dimensions is how the wrong one gets bought.

A template stores what a box can hold. Using that as the parcel's weight would quote every shipment at the heaviest thing that box is rated for, so the dimensions come from the template and the weight stays yours to enter.

Carrier label links expire. The label is fetched fresh when you ask for it rather than saved as a link that rots — because a dead label link is discovered at the printer, by whoever is holding the parcel, and the workaround is buying postage you already paid for.