Anyone in a block swap deals with this question. A lot of it depends on how much you want to fold the block. If you can get it down to the size of the envelope, go for it, just make sure you have it weighed at the PO AND!!!!! write on the envelope NOT MACHINEABLE. I have gotten some in the mail where the envelope was trashed. For that reason it is important to put the block in a baggy or plastic wrap and put the address (label or paper) on/in the plastic. I have gotten one back that way. Better be safe than sorry.