When I was a kid I went through a stamp collecting phase. I went to Mount Pleasant PO and bought, for some reason, some "to pay" stamps. Lovely they were. Thing was, it cost me the price to pay for the stamp, ie: 10p to pay would cost me 10p.
A few years later, I'm 16 and job hunting and sending letters all over the place, I had used up all my silver jubilee stamps and other decimal ones so I turned to my to pay ones.
I have no idea if the recipients had to pay on receipt, I didn't get many replies.