I just now sent a message thru to a customer with the word "invoice" in it and it went thru OK.

So not too sure what combo of words they are looking for.

 

This was a customer looking for a special order of not listed items and I just said that "...since we already have your payment details from your previous order I can send you an invoice".

"I will send you a P@YP/\|_  1N\/01CE shortly" would be one way to get around it if you don't mind your buyers thinking that you've had a bit too much to drink. Smiley Indifferent

 

Otherwise, maybe an intentional typo or two would do the trick; surely, most could guess what a "PoyPal invojce" would be...