Yes to all of #1 for the subject line of an e-mail.

Mind you, 'innvoice' is a nice pun for a hotel bill...

:-)

escoville calls it a 'hotel bill', not invoice. I think xe is right. Invoice implies a formal document issued by an accounting office. If you are paying it is more likely to be a 'bill'.

Re #5: I don't find that distinction to be at issue here (if it's even valid -- M-W, AHD, Collins, and Oxford treat "invoice" and "bill" as synonyms). "Invoice" may be a tad more formal/more technical than "bill," but they are basically the same. You can pay an invoice. You can pay a bill. Nor do I believe that escoville, if I am reading the post correctly, was trying to correct the use of "invoice" to that of "bill" -- it was simply a way of pointing out that "innvoice" is a humorous pun when it comes from a hotel. (It thought the same thing when I saw it.)