There are pumps in service that are intelligent enough to accept QR codes (like Dresser Wayne & Gilbarco)... Mobile uses this type of technology for their speed pass system... several brands offer loyalty programs that are fairly sophisticated...there are also pumps that can be retrofitted with a side box for passing payment codes to the POS system.
Besides the cost and durability challenges (would you stick a computer outside in minus 20 temp & freezing rain?) you have to deal with proprietary POS systems (the largest being RUBY)... these systems have to be able to map all types of transactions that are unique to the petroleum industry (ie underground fuel storage inventories, fuel shrink, agriculture tax discounts etc).... hence very little competition.
You simply can not afford to have delays at the pump with 4 cars stacked at each island.
If the demand is large enough it will force their hand... it took 8 years for Ruby to come out with an upgraded model...
I wouldn't count on the Petroleum fat cats to be early adapters.
The best way to start is to bypass the pump and go inside for your payment (they also accept cc inside)... then you can have a small device attached to a COM port and map the transaction as a separate payment key
btw, never blame a convenience store for the cost of gas... it is so competitive that they are lucky if they make 10 cents on a gallon (after numerous taxes)