Cashback websites act as referers and use the promise of cash back or other types of credit as incentives. In a sense they advertise your site and get a commission on the sales they bring. This means the Sam and NP would have to raise prices 20%. They give the website 10% commission on each sale they refer. Additional they may give an extra 10% to the customer. But more times than not the refer gives a portion of thier commission as a discount to the customer. So they get 10% but offer the customer a 7% and still pocket 3%. So perhaps Sam would only have to raise prices by 10%. I probably rather just get a gold membership instead.