Web Receipts folder!
I always save web receipts as PDF instead of printing them, because I really don't want to end up with so much paper. I created a folder at first, but one already comes with OS X. Even better, the one that comes with OS X has a one click shortcut for saving to it. When you go in the Print menu, you just have to choose "Save PDF to Web Receipts folder". The Web Receipts folder is located in the Documents folder.