So I forgot that someone who ordered a WL book from me also asked for an upostable freebie I had listed. No problem except I forgot to add the weight to the postage.  Can I just add some stamps to it or will that cause it to be sent back to me?  It's going 1st class.  I paid the postage fee so I wouldn't have to go to the PO.

Stamps with trackable postage is OK. It is only when there is only stamps that you need to take it to the PO. Now there is no guarantee that the book won't pass under the nose of someone who doesn't know that and you get it back anyway. But, as the regs are you should be fine to add more postage and drop in a blue box or mail from home.

I'll give it a shot. It doesn't need much more postage that what's on the box.  Hopefully it won't come back.