Don't see why it wouldn't be safe, but the (few) others from the US who I recall posting about it on here over the years usually say they're hard to find over there, few locations selling them, and maybe in a physical format or something. So if you can find them, should be fine. Here you just get them at machines where you can also pay for a lot of other things, like bills or concert tickets (well, less of that now...) or recharge prepaid phone cards, which are placed around hypermarkets, in shopping centers, supermarkets, metro stations and even just on the street, in intersections or near bus stops.

Circle-K (previously Statoil, previously BP) have them here, and they're a Canadian company.
I would assume that means they're somewhat prevalent in general in North-America. Check with them.