That's interesting, I filled up GPL many times in France and have yet to find a unmanned pay by card outlet yet, but I have found pay before you fill ones

, so you have to guesstimate how much you need and pay upfront. Don't think you get a refund if you overpay. And on GPL on the Autoroute services where you had to prepay I asked 3 people where the prepay was and still couldn't find it, so left