When I worked in 'the industry' we used to supply a promotional points company with Apple devices. Their members would earn points by shopping with certain suppliers and then they could redeem them for iPhones - iPads etc.
I believe they were sent out with a "Thanks for redeeming 10000 points letter" rather than an invoice as such - as we invoiced the promo company for the $$$ while they deducted the points from their member.
So could be something like that?