Not sure when you are going but I went through Priceline for 7 days, standard size (which is a full size car but with two doors) with
Avis for 17.00 per day, with taxes and surcharges and Pricelines Processing fee of 5.95, came to 162.63 total. This is for mid September. This is the third time I have used Priceline and we are extreamly pleased with them. Last time we bid 15.00 a day for a mid size, that was accepted and upon arrival at Avis' counter I requested any color but white ( I HATE white cars), the woman was quite nice and asked if beige was ok and I said sure, well, we ended up with a nice upgrade to a Monte Carlo. GREAT car.
The only problem with Priceline is that you cannot shop for prices, cannot cancel and sometimes you may end up off airport. Of the three times we have used them we were only off airport once.
They only use the top companies, IE:
Budget, National, Avis,
Alamo (I forgot the other one). For me, this works.
One hint, if you should use them, bid low.

After my bid was accepted I checked the Avis website for the same car and we really saved a lot of money.
If you take the rate I received minus the processing fee I am paying 22.38 per day for a standard size car. You are paying 26.87 per day( does this include all the taxes and surcharges?) for a midsize car and taking a chance that your coupon will be accepted (sometimes they say they run out of upgrades) and going with a company that has not had the greatest reviews here. I have never rented from them, however, I did make a reservation with them and got a run around, rude reps and a computer virus in an email (luckily I have a great antivirus program that scans my mail and one I keep udated). For me, this was a no go.
Wow, this was long, I am sorry.
Hope I have helped.