I've spoken to Matthew about this before;
Read here;
How do I get a generic Sythe Prizebox by donating?
To get a generic Sythe Prizebox, you must be a $100+ donor and then donate an additional $50 or more. You'll get a Sythe Prizebox for every additional $50 you donate. So, if you're a $100 donor and donate $100, you'll get two Sythe Prizeboxes! Please note that the additional $50 must be in the form of a $50+ donation. If you donate $30 and then $20, you will not get a Sythe Prizebox.
So when you were a $50 donor, you didn't qualify for a prizebox until you became a $100 donor, so the first $50/$150 you donated was ineligible for a prizebox(I know u made 1 large payment, the system treats it like its multiple $50 ones). So that now means you should be eligible for two prizeboxes, however;
I've had the issue where if you move from a lower donor to above $100 then some($50-> $300 donor for example) the system takes the 6 prizeboxes ud normally get and always subtracts 1(So ud receive 3 for going from $50 to $300, not 4, but if u donated $250, then another $50, ud get 4.).
Its something to do with the fact it was all donated at once vs in increments, its always better to donate in increments and you passing the $100 donor mark on a single large donation, this has been the subject of quite a few support threads in the past to.
To my understanding the above is why you got one.