You need to understand that there is Priceline and then there is Priceline, and that there is "pay with DC", and that there is "pay with DC".

Priceline is no longer limited to "name your own price" (blind bidding). If the new kind of non-blind-bidding Priceline isn't prepaid, and you can indeed pay the rental car company direclty with DC, then you should be ok if that's the kind of Priceline you use. (I have no idea if that's how the new kind of Pricleine works for car rentals. I only know they're no longer only "name your own price" from ads in which they only talked about hotel rooms.)

The problem is defining who you have to pay "entirely with DC" (to be eligible for the benefit). In the case of blind-bidding Priceline, AFAIK the rental car company doesn't see your DC number, only Priceline does. You pay Priceline with DC, but the rental car company sees a "voucher" or whatever from Priceline as the form of payment. One lawyer might interpret that "paying entirely with DC", but another might interpret it as "paying entirely with Priceline"!

It sounds like whoever was giving the info at the Benefits Assistance center was simply reading from a script. That's not sufficient to resolve this is issue of whether the benefit applies if you pay for the entire rental with your DC but to someone else than the rental company (and thus the rental company doesn't show it as a DC purchse).

If you're that interested, I suggest you call the same number, and kick it up until you get a supervisor who understands your question and doesn't read from scripts.