Originally Posted by
LessO2
I didn't see where the OP said he/she bought the stamps with a credit card. It also reads to me the postage was self-administered, bot via someone at the PO.
I am not aware of any serial numbers on stamps, anyway (the ones you stick on yourself, I mean).
He was making the point that any mail that has postage paid by meter or postage paid online is not subject to the rule since you know who applied the postage. The rule only applies to stamps.
When he pays online, he can leave the package wherever.