This may be specific to Avis Switzerland? I've not heard of it anywhere else.

Here in the USA (and elsewhere?) you can get as much as "up to 35% off" (with no "bi-annual fee") simply by using the "right" AWD codes (from airlines, hotels, and others). "Up to" is complicated to evaluate, I presume it varies by location and how booked up they are and stuff like that. All I know is that it gives me significantly better prices when I use the coupon than Orbitz (or some other third-party site) shows as a the prices without any AWD.

I'm pretty sure that the discount from the "on the road" program and any AWD you might use will not stack, rather Avis will simply choose whichever AWD (the one you specify, or the effective AWD that the "on the road" program provides) that gives you better savings. If you buy "on the road" to get "up to 30% "off but then you use a free AWD to get "up to 35%" off, it may end up using the free AWD and you getting the same savings as if you had never bought into the "on the road" program.