Well that was one confusing and angry answer...
OK, Fact one, postmen DO NOT collect money under any circumstances. If anyone has anything to pay for any reason they can either do so at the sorting office, or put the value of stamps on the notification card and it will be delivered.
Personal mail that has to be returned is just sent back, no charge. I don't know for sure, but I should imagine a charge would be made to business customers if a lot of mail has to be returned. A lot of junk is sent out, and a lot may be returned for whatever reason (not known, no such address, gone away, refused etc) and it is all extra work on top of the original mailing, so a charge would be justified. As I said tho, I don't know for sure.