I don't know the specific one you are looking at because the ones I do have a different fee, but here is a general scenario. You arrive, pump gas either paying inside or at the pump depending on the shop and then go inside for a small purchase to evaluate the cashier friendliness, upsell, etc. Outside again, pick up your camera and the letter of authorization that you present to the cashier. From there it becomes a picture taking open shop. With some you may need someone at the station to sign off on your check list of observations. With some you may be leaving a copy of the check list at the station. You make sure you have all the photos you need before leaving the location to go home and upload receipts, photos and the report.
These are not difficult shops. They can become tedious if the cashier or store manager decides to do a song and dance number on you that this or that the 'last shopper found was fine', 'the county won't allow me to do' or any manner of malarkey. They can be amusing as you watch them scurry to try to get into compliance.
For $12.50 right next to home it could be an okay trial to see if the shop works for you. Generally I don't do them at stations I can regularly mystery shop because once you have revealed yourself as a mystery shopper you pretty much have to wait for a personnel change before you can shop the location again.