Anyone familiar with Post Office rates - ebay postage purchase

[ckso]

I sold something on ebay that according to my scale is 12.5 oz. I'm hoping it's accurate and that I can send it first class which would be cheaper.

If I purchased/print the postage label and it turns out to be over 13 oz at the post office, can it be changed to priority and i pay the difference? Or would I have to buy a whole new postage??

I know I can wait and purchase there but it's cheaper online and offers free tracking.

Anybody encountered this or have suggestions?

[Avispa]

if your post office is not too far you could go and use their scale to make sure you're under 13 oz and then go back home to print your postage online. I know that if you are over but still within first class limits (under 13 oz) they just charge you for the difference. I don't know what happens if it bumps you to another service though.