question about the mystery box

how does the box x work? if i buy 2 of them and 1 box a, is the grand total 150 or 250? is 1 box x + 1 box a = 100? or does 2 box x + 1 box a = 250 work too?

Box X was $50 when purchased at the same time as Box A or Box B - totalling to $100 for both A/B + X. If you had done all 3 boxes, your total for everything would have been $150 + shipping.