I also use the cling wrap and press out all the air. I use business envelopes for both my F8s and the return blocks. My PO is pretty good with taking them. It cost 45 to mail the F8's and 65 for the return blocks for the US. Iknow what you mean about some PO's. Last boom, half the envelope was missing on the return block from luv2sew, the only way I knew it was from her was the city/state on the post office stamp on the upper right hand of the envelope. She had it wrapped in plastic and it was perfect!
I also make a note on the spreadsheet which F8 I sent to everyone.