Easiest way to track sale referral
 

I am looking to sell a service through paypal. I want others to be able to promote the service and if they make a sale, to receive some extra service for free from me.

So basically, they should have their own link, for example domain.com/a/user that would redirect to paypal checkout. If the person going through the link buys the service, I would be notified (by email preferably) and I can than give the referring user something extra for free as a thank you (will be done manually).

Thank for

Assuming you would be the seller;

Use the IPN Postback at Paypal and script the buy page to add a hidden field in the post to Paypal include the refer's ID.

When the sale posts back you do something with the referrer ID posted back.

This has to be programmed and there are example INP post back scripts in various programming languages at Paypal.

https://developer.paypal.com/docs/classic/ipn/gs_IPN/