To your specific question about fast food, they normally pay between $5 and $10 fee and reimburse you for specific purchase requirements. Some require you make a purchase both inside and in the drive through. There are fairly simple cleanliness observations to make and some very specific and critical timings of service that vary greatly from client to client and company to company. They all require you check the restrooms and most have time requirements for an inside dining in that vary from 10 - 30 minutes. Some prohibit you taking along anyone else,others don't specify but they don't reimburse the meal for your companion.
They all require that you report your shop on line fairly promptly. They all require that you submit your receipt(s) to show you were at the right location at the right time and ordered the required items. These usually are scanned and uploaded into the report, though some allow you to fax in the information. In most cases you are spending more than the fee but will be reimbursed if you have followed directions. You will be paid/reimbursed anywhere from 2 weeks to 3 months depending on the company you are doing the job for.