Two words: Save PDF to Web Receipts Folder.
Obviously. This is what I do all the time in order to record my on-line purchases and keep track of them without actually having to keep track of them--I print the receipt as a pdf and store it in my 'receipts pending' folder on my desktop. wha? you don't have said folder? you don't normally use words like 'pending' in your own personal space? well sure. but now the glorious people at Apple have added the above selection to the drop-down menu in all print dialog boxes. It's so exciting. I can't tell you. go try it now. super fun. go buy stuff so that you can try it. very cool.
and you know what? it doesn't matter that it just automatically saves it under whatever random name the webpage has. that's what spotlight is for. huzzah.