It's a real faff but not too bad when you know all the wrinkles...
Firstly, you're over 3.5 tons so you would have to buy a GoBox to travel on Austrian autoroutes and expressways.
How much toll you pay per km depends on the Euro Emission Category (EEC) that they have recorded for your vehicle. The worse your emissions, the more you pay.
The cheapest toll rate will be for Euro 5, then it gets more expensive for classes 4, 3, 2 and 1.
The chances are that, on your older van, it will be Euro 4 at the very best (but more likely 3 or 2, sadly) so it will be more expensive tolls.
So, the question is - what's your van's EEC?
Well, it may be declared on your V5C but, if not, you need to write to the manufacturer (Fiat, Mercedes etc.) and request a emissions certificate (it's called a homologation letter) stating what EEC the engine conformed to at the point of manufacture.
When you buy a GoBox on entry into Austria you need to declare you EEC.
If you are EEC 1,2, or 3 then you simply declare it when purchasing the GoBox and that's all you need to do.
If your EEC is 4 or 5, which get preferential rates, you have to prove it to them by submitting an application form (I've put a web link to the form below) to the GO authorities, plus copies of your supporting evidence (homologation letter and copy of V5C).
You can send these documents (post or email scans) IN ADVANCE (up to 6 months) so it's all sorted when you subsequently buy the GoBox.
Otherwise you have to post/email them within 14 days of buying the GoBox
. This is where we went wrong and got fined
BTW in the summer, when the mountain passes are open, with a half-decent satnav and plenty of time, you can avoid the toll roads completely. And discover some lovely Austrian villages in the process
Application form for Euro 4 & 5 classes
Hope that helps