Oh there's a fun legal problem at play here.

Depending on the region its likely fine to only accept card for goods being offered.

However it's not fine to only accept cards in a scenario of debt. (The idea you've consumed or changed something before paying)

So even with all this signage if the pump allows you to pump before payment. You'll need to accept cash as the business or be willing to provide them an invoice to be paid out later through the banks.

You might argue that the fuel is in the tank just take it back. However, this will not be the same fuel that is being sold. As it's now had an opportunity to be contaminated by whatever was in the vehicles tank. Taking that contaminated fuel and putting it back in the main holding tank would potentially be considered as fraud.

Edit: so many labels I thought I was looking at a gas station self serve haha not a grocery store.