I think if you offer to pay for someone's coffee, they should feel free to order any coffee on the menu with the exception of

a) that really expensive Kopi Luwak (or Civet Coffee)

b) coffee that features gold leaf

c) coffee with an expensive shot of alcohol in it.

If the person treating isn't ok with paying for whichever size or flavour of coffee the other person wants, they should have

1) not offered to treat; or

2) chosen a cheaper coffee shop to treat in.