online printing services with referrals programs

[Jocker]

Hi,
I'm a software developer and I would like to insert into a freeware programs of mines the possibility for the user to print his photos from one of the many online printing services. I was wondering if one of these services offers a 'referral program' (it pays a small percentage to the referrer of the new customer).
In this way I could keep my program free and earn a bit from photo orders.
Does anyone know some services that offer this opportunity?

thank you