Austria move

As a follow up to this post,I’m interested in finding out costs,‘‘tolls’’ across Belgium,Germany,Austria, Hungary and Romania,in my 7.5t.
Also what method of payment would you recommend for a one off trip,my destination is Bulgaria to bring back our house contents to the uk.
I did find the Austrian site easy to access and find the cost,‘‘87 euros one way,Merc 814, euro2’’,plus you can call at the services just into Austria to get the ‘‘go-box’’,that seems pretty straight forward.
Can you just pay adhoc at relevant services as with car vignettes?..don’t mind if it’s a bit dearer,it just seems less hassle than having to send for boxes for the truck.
I have done this journey twice before,‘‘in a car/car and caravan’’ so the route is not a big problem,it’s really all motorway apart for Romania.
I would appreciate all comments and help on the subject as I know many of you guys have vast experience of such journeys.
Thanks for reading.