Is this listed as a "mystery shop and revealed audit" or just a "mystery shop" (there are a few that are pure audits, but they're dead easy)?
I think I've done this shop, I vaguely remember one where the gas was $1 and $1 inside. (Most are $5 or 2 gallons of gas)
They're not hard, but if it's the pure mystery shop there is a lot to remember. The audits are actually easier the first time, because you're buying gas like a normal person, then you go in and check the restroom like a normal person. Then you buy a soda or candy bar at the counter like a normal person. Get receipts from both the pump and the cashier (not like a normal person; you'll usually have to ask for it in the store). Then you go out and drive around the corner of the building to make your notes on the survey form. At that point you'll see if you forgot something in the mystery shop part. If you forgot the inside receipt, you go back in and ask them to reprint it. Then you give them the letter and tell them you're doing the audit.
During the audit, you can double check anything you forgot to notice during the mystery shop part of the job.
For the audit, you have a step by step list of things to look at, things to take pictures of, questions to ask. So that part is easy and you're not trying to sneak around and hope they don't notice you noticing that there was spilled water all over the floor somewhere.
The gas station pure mystery shops aren't hard, but they go pretty quick and there's a lot to remember. If you've done a few audits first, your eyes will be used to looking at the window washing supplies, checking for credit card applications, looking to see if trash cans are clean and not full, note if the cashier is in uniform, remember the name and description, etc. So use an audit to learn the terminology and figure out where things are. It will make it easier to automatically check for them when you won't be able to double check while reading the instructions on your clipboard.
I blew my first-ever gas station shop/audit because I forgot to get the receipt from the cashier and didn't realize it until I went home and I didn't have it to upload. Luckily they let me re-do the shop the next day. Since then I have had to have several reprinted. I'm just too busy trying to memorize his description and name to think about asking for the receipt. So I go back in and say, "I forgot to get my receipt and my boss won't reimburse without receipts any more. Can you reprint it?" and they have always been able to reprint it. But I'm getting better. Although sometimes I have to go back in and ask a question to get another look at the cashier, though, because it seems I just can't hold that many facts in short-term memory any more and if I ask for the receipt, I forget what color his shirt was.
Time to build a bigger bridge.