As the owner of a light diesel SUV that runs at less than 5L/100km on the motorway I'd be happy to pay at the pump. Pretty sure it would work out cheaper for me.
and reduce your admin, of having to remember to buy road users at whatever kms you're up to (as the owner of a 3l diesel)
I've never really understood why people consider RUC renewal to be an administrative burden. I just accrue for the cost each month based on an assumed annual mileage, and then when I have exhausted my previous prepaid mileage it takes less than five minutes to go online and pay for the next allocation.