Q: can't buy more than one card in photos

At least we know now, that Apple is aware of the problem and will hopefully fix it.

Are you using the latest Photos release in OS X 10.11.5?

I am on OS X El Capitan: Version 10.11.5.

Yes, there seems to be an odd — really odd — glitch in the system. Everything works perfectly if I only want to order 1 copy. If, however, I want to order more copies from Photos (I wanted 9 copies of a card), it doesn't work at all.

It's so odd.

I do not need to order cards right now, but on my Macs it is working:

When I click the "+" button, the quantity increases:

I hope it will soon befitted for all systems.

I checked right after you wrote your post, but I'm still getting this when I try to increase my order (i.e., I can't order one copy of a card at a time in Photos).