You have to spend $175 then you get $25 off. This means a maximum of 14 % saving. But you get additional savings on top of that when you use the pay now option. These elements are part of the spring sale at Avis.
Just checked out and booked a two day rental at DTW using the spring sale from Avis. Including service fee and taxes the rental rate is around $31 per day for full size. A great deal from my point of view.