ON that AJ hand I think that's a great spot to bet the river. If he has AK/AQ he had you beat anyways, and I can't imagine him slowplaying two pair that far. His hand should be either the flush draw or a lower ace.
The lower ace will probably check behind a lot of the time, and the flush draw will bluff occasionally. we can assume he's not tricky enough to do anything but fold the flush draw (unless he ran of with
[Qh]). He could also have
xh, which will call a lot of river bets but never bluff now that he's made a pair. the same could go for a lot of the
xh hands as well. They're going to call but shouldn't ever bet.
So I would be $12 on the river here. There's no reason to keep up the hard pressure given what he's representing. The draws have missed, so you're not protecting your hand any more. Not many people at 50nl are going to be making river bluff jams on you, and unless you think he's one of the very few, I wouldn't worry too much about that here. The $12 bet will get you a call from every ace, and lots of flush draws turned into one pair.
The check will induce a bluff from flush draws that haven't become a pair on this spot from some players, but certainly not all all players nor every time.