I just received payment for 2 of these shops I did earlier this month. Gas is under $5 in my area so I expected to be reimbursed the $5 for gas and $1 for the item I bought inside (I spent like $5.50 on gas and over $1 in store to max the reimbursement). The editor changed my reimbursement amount from $6 to $5.41, which is $1 for the in store item and the price of 1 gallon of gas.
I did another shop this week and the amount approved by the editor is the correct $6. I sent an email into ipsos never never land asking for the rest of my money (I had 2 shops affected) and will let you know if I get a response.