They will charge your credit card accordingly after it is weighed by them. Otherwise, if you drop it off, they will weigh it & correct the postage on the spot.
I've turned to FedEx (usually 2-Day). Comparable in price for us & better accountability. Last year the USPS Priority mail took over a week to deliver some cigars to a bordering state (Pa.). All they could do was scratch thier asses in unison when we wanted it tracked.