Oil and gasoline will continue to be moved and distributed no matter what the cost per barrel is, so I don't see where it will affect GLP margins. It may even increase usage if the price at the pump falls far enough.
The average profit on a gallon of gas at the retail end is around 5 to 7 cents per gallon. This price is of course adjuted frequently by changes in wholesale prices. Gasoline sales aren't really a big factor on the bottom line on the retail end, That said, you do have to have good volume to attact people into the store which is where most of the money is made on pop, cigs and coffee etc. Lower gas prices might actually produce a spike in revenues at the retail end because of increased traffic in the stores.