I don't think I would do any gas stations for $4. No, I don't think, I know I wouldn't.
I do about 800 gas station shops a year, on average. Most of my regular shops are one of two types: mystery shop/audit and straight audit with a few that are straight mystery shops.
The straight mystery shops are two types:
(a) $8 and reimburse for two gallons of gas (up to $10 total) and $2 in-store purchases
(possible total of up to $20)
(b) $6 and reimburse for $5 of gas and $1 in-store purchase (total of $12)
The mystery shop/audits (three different ones) pay as follows:
(a) $10 plus $1.00 reimbursement in-store (total of $11)
(b) $10 plus reimbursement for 2 gallons of gas (up to $10 total) and $2 in-store purchases
(fee increases to $15 for night shops) total of up to $22 (days) / $27 (nights)
(c) $10.50 plus reimbursement of $5 in gas and $1 in-store purchase (total of $16.50)
(d) $11 plus reimbursement for $5 gas and $1 in-store purchase (total of $16)
The audits pay a straight fee of $14.50 per shop.
I'd much rather do the straight audits as the forms are provided by the MSC and the photos are pretty straight forward but poorly maintained stations could have upwards of 35-40 photos. The mystery shop audits are good because I can get gas along a route and get reimbursed for the purchases especially for snacks which I would probably buy anyway. I like to get a good mixture of shops for long routes so that I can get enough gas to get there and back and perhaps have some left over for future trips. I currently have nearly 90 gallons of gas in my garage from past shops that will keep me from having to buy any gas for the next couple of months until the next quarter or period's stations are released.
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One buzzard to another while circling high overhead (paraphrased), "Patience hell! I want to shop somewhere."