PayPal should offer this service for designers to prove services rendered.
PayPal could fix it to where a business can upload their design to a specially made PayPal account for each customer, the account will hold their image and code, a person can view the finished work on PayPal as a resized image with a watermark, download the image or code from PayPal after they pay for it, and this would be proof of the transaction and quality of work/service offered by the designer since everything is on PayPal servers.
It just makes me wonder why PayPal doesn't offer this kinda service, maybe they haven't thought of it, or not in their best interests since they would have to invest to get it up and running.
Anyway, designers should keep an updated portfolio of their work, this could help prove your service, but will PayPal accept it?
Paypal doesnt gear their business towards designers, they gear it towards auction payments right now. There is also not profit to be made by paypal using your method, just a bunch of webspace they have to support. Not to mention the technology they have to program to get this thing up and running.
Good idea for designers, but paypal will never do it, its just not profitable.