There doesn't seem to be a way of paying these. Anyone got any experience of them? Likelihood of being fined?
Post office. Or you can also register online, providing your numberplate, but I guess the post office will be easier. The site was a bit of a mess.
I was initially amazed at the process (being Portugal) but ended up hating it all.
I was travelling by car (UK rental).
Passed the first gates, asked at a service station, they told me about the prepaid cards, etc.
When I got to the hotel I registered online, bought 25 euros of credit but I was unable to "pay" for the tolls I passed already. There was no trace of them at all on my account.
Ended up not paying the initial usage, travelled for the rest of the week using my 25 euro prepaid credit and expected to get fined once back in the UK, but I never received anything.
I don't know if they actually got money off those 25euros to pay for my earlier tolls, as there was no indication of it at all.
Anyway, easiest thing: register online and go for the prepaid thingie.