That’s actually a good point :) how would you see this working? Your affiliate gets a unique promo code for your checkout, then if a sale is completed with that promo code, it attributes the referral to that particular affiliate?
Last time I looked at your competitors nobody seemed to be doing it, so I implemented a halfway version myself. I currently generate a unique Stripe promo code for each new customer. If someone uses the promo code it gives them a small discount and gives the referrer credit against their account. Its not very popular as it stands.
If you have any advice, or know of any existing SaaS that can handle this, please do give a shout.