Best thing I would suggest is do something within the site it self, not paypal, for instance Im a part of site that you pay for people to help you out/fix things on your xbox or gamertag/ in game stuff or just buy stuff from them.... whatever it is, you then pay them through paypal.(this site does it by donation but this is NOT the point.)
(THE POINT)>>>They have a way of being able to go to someones profile and leaving feedback and you can select multiple options:
Based on the screenshot I captured for the site I am affiliated with I kept in mind the privacy of the site and the user I user basically as a dummy to show you what Im trying to expalin
ROLE:
1) the buyer
2) the seller
3) trader
This could be changed to something like
Referred Product Bought:
Then in a drop down menu contain all of the folllowing and more.
1) Macro
2) Ragnorak
3) Ravin
4) Thor
5) Tyr's
6) DIY Rag.
7)PS3...
[the list could go on and on with every viking product, present ....and future]
Prevent anonymous accounts from boosting peoples referrals
In order to prevent users from making bogus accounts to get there referral # up have another section that is a fill in text field where the person inputs the Transaction ID of there order(yeah its long but i feel it would be the safest way to prevent "referral boosting"........HOW.....
UPON SUBMITTING THIS MINIATURE FORM, THE FORM WITH EITHER BE SUCCESSFUL OR FAIL BASED ON WHETHER OR NOT THE TRANSACTION ID IS VALID OR NOT.
Have a part for which consol it was for:
Have buttons distinguishing which console it was for ~Xbox 360 ~PS3
Then have a simple comments section for the user doing the referral to add what ever comments they want.
****I WILL EDIT WITH PICTURES OF WHAT I MEAN TO HAVE THIS ALL MAKE SENSE
AGAIN THIS IS JUST MY .02
Heres how you get to the leave feedback page on the other site I'm apart of. This layout could be used to create vikings own layout from what Ive derived
Click the first picture below to see all the picture in order with explanations on each.
The leave feedback page (I included notes to how a form may be create for viking's site)
How you would get to the referral for a user on vikings' site