Maybe the system doesn't classify photo paper and copy paper as office supplies.
I'm 100% sure I've had the $10 off 50 work on photo paper in the past, and I think it has applied to copy paper too. (Unless they've changed something in the past few months?)
It may have been the attempt to apply 2 product-specific coupons (15% paper and 10 off 50 office supplies) to the same items that tripped me up. Maybe somebody will try only the 15% and the 10 off 50, and see what happens.