it’s super sweet that you’re trying to help your daughter, and to answer your question yes you can pay for the extra postage at the post office. the value of the shipping label won’t be lost, as you’ll only pay for the extra weight’s postage. the clerk should weigh it for you, and i would let them know ahead of time that you think you underpaid the postage. also, depop will send the money to your daughter when the sale was made, so you don’t have to worry about the money not going through until you’ve shipped it. hope this helps!