Very important to get the tolls right in Portugal and keep any receipts.Any fines are collected by the scam scum EPCPCL. See this BBC Watchdog report
https://www.bbc.co.uk/programmes/ar...pTPX/watchdog-european-parking-collection-plc
There are many pages on the internet about their behaviour and Hungary and Portugal seem to figure very regularly in the complaints.
I believe they get photographs of vehicles from those countries and send fines/invoices sometimes years after the event.People have either not kept all the vignette paperwork,forgotten,or accept they were at fault and pay up thinking it is the end of the matter. Nice little earner split between EPCPCL and the relevant country.
I received 2 for a journey in Hungary demanding £200 for an offence that had not taken place.Luckily I had kept every piece of paperwork and eventually they accepted that I had bought a vignette and there had been "a technical error"
Many posters say not to pay as these invoices cannot be enforced in UK but it can be very worrying and I'm sure lots of people just pay up even if they have not committed an offence.