I was printing a bunch of labels today. Usually I don't check the weight as I use Media Mail and it only needs to be under a pound, so whether it's 6 oz or 12 oz is irrelevant. However, the listed weight for one book was so off that the label printed with First Class postage. I didn't notice it at the time because it's the exact same price as the Media rate ($2.66). 

Obviously, my book is too heavy to ship at that first class rate. What are my options here? Any way to get a refund and reprint?. If I bring it to the PO can I change the class of the mail or add the difference in postage? Can I just add stamps to it to bring the price up to what it should be?

Thanks for any advice you can give!

Easiest fix is to cross out the words ‘first class’ or put a blank label over it and write media mail.  If you’re have full Adobe Acrobat you could fix the actual PDF.  I print postage with a dymo label maker at home & it all says first class; it doesn’t really matter on the postage itself so much since you’ll notice many stamps have it too. The postage at $2.66 is correct so I don’t know why it did that. Maybe a glitch - you could report it if you wanted to.  If it were after the postage goes up in a couple weeks you could use stamps or get postage at the automated postage machine to make up the difference.  As far as cancelling a label, sometimes PBS used to be able to, but it has to be requested almost immediately after it happens and I don’t know if they can still do it.