Hi Bill-I'm only out by myself to nursing homes a few times a month. I also play guitar/keyboards with a Irish group in town. http://www.ceolnaheireann.ca/
Years ago I played with a 4 piece group-for various service clubs,weddings and so on. Everyone wanted to get paid when we played, so we generally asked for cash and of course would sign a receipt if requested. Most of the nursing homes in this area simply give me a cheque-some will give cash & for those I sign a receipt. As far as they are concerned, a cancelled cheque is their receipt. I have no objection with sending an invoice -it's just that I've never had to do it.