This one’s really simple: you start with some two-digit number 10X + Y. Then you subtract X and Y, leaving you with… 9X. Thus, they know you’ll pick a box which is a multiple of 9. And, of course, all the multiples of 9 have the same gift in them.
ETA: Not that it matters, but they even make boxes 90 and 99 have the same gifts as all the other multiples of 9, even though they could get away with changing it up, since X is guaranteed to be in 1 through 9.