I love I had to scroll and read right to the bottom to find the perfect answer. Installed Invoice Wizzard - it's exactly what you're all looking for. (invoice download button, rather than attaching as a pdf... but that works for me)

In godaddy - you can add a section called 'website / html' - that enables you to paste the 'buy now' code within... but, having a play now, and bth - I can't get it to 'save/publish'... so not 100% sure this will work.