A couple of things. First change the client name to 'gas station' because you are naming the client and the company in the same thread. There is a little blue edit note in the lower right hand corner of your post to do that.
Second, it has been a long time since I did them but I believe they reimburse $1 in gas and up to $1 in the convenience store for a magnificent $2 total reimbursement max. When I have done these shops I have simply carried a gas can suitable for filling my lawnmower because it is silly beyond belief to pull in with a gas hog truck and buy $1 worth of gas. You could buy more, but why when the price is a nickel more per gallon (sometimes more) than nearby stations? (after all, they have to pay for those shops somehow)