On the Print sheet there is a button labeled PDF. Click that button and you get a drop down list with several options. (The options you get depend on the version of Mac OS X you have.)
The "Save as PDF..." option will create a PDF file and ask you where you want to save it. If you are using your desktop as part of a GTD strategy, save the PDF to your desktop.
If you choose the option to "Save PDF to Web Receipts Folder", the PDF is created in a folder named Web Receipts inside your Documents folder.
Once you have received your purchased items, either print, file, or delete the PDF file that you created.