What's to prevent them from responding to this by asking for an invoice? (Note: If your answer is "well, send them an invoice", the problem is that you'll prob

What's to prevent them from responding to this by asking for an invoice? (Note: If your answer is "well, send them an invoice", the problem is that you'll probably have to contact a lawyer to make sure that the invoice is legal. You don't want to fraudulently imply a preexisting contract or run into some other legal gotcha that you wouldn't even know about.)