You wrote "was wondering," so perhaps the "rule" calls for saying "policy was." But common sense (and probably most native speakers) would use IS. You want to know their current policy (right?). If you used WAS, that could refer to a policy that they USED TO have.
Both sound fine to me, although I think 'is' is more common.
But if someone walks into a store and says to the salesclerk, "I was wondering what your credit policy was", the salesclerk is not going to think 'Oh, she is asking about last year's policy'.