@WasCy – I just realized that myself.
Mine was 9. I noticed that if you added up the digits of hers, they also equal 9. I thought of another – and inevitably, that also added up to 9 (the two digits).
I thought that was the trick, but “99” is an answer as well, and that equals 18 when added up (although add that up, and it equals 9 again), so @WasCy has it with the multiples of 9.
It also regenerates with a new gift each time at that number, so it’s not obvious (i.e., since the chart changes, you’ll get a different gift in the 9 multiple box).
I’ll note that it’s super-transparent if you realize that subtracting the two digits from a two-digit number means that the highest choice should be 81 – 99–9-9.