I've been faced with that situation a number of times. I always ended up taking the time to take the incorrect postage label off, cancel it for a refund, and put a new label on.

If it is a small first class thing, I think you could just put on a 20-cent stamp for each ounce you are short. Simple, and they wouldn't bother you. I'd also write about the weight difference on the envelope/mailpiece.

However it is complicated. I just made this post on the eBay Shipping discussion board:

Say it's a minor matter . Say you printed and affixed the postage label (first class parcel) for 3 ounces. But it turns out to be 6 ounces.

(1) Can you put on extra postage stamps to make up the difference?

(2) If yes, should the total postage come up to the retail amount (what you would have paid a postal clerk for the entire shipment)? Or have the total come up to the commercial/online amount if done correctly in the first place?

Note: online parcel rates: 3 ounces $1.64, 6 ounces $2.15. ...

clerk/retail parcel rates: 3 ounces $1.95, 6 ounces $2.46.

Thanks,

Robert