Hi Vlade, welcome to the forum. Most mystery shops and audits don't pay much per shop. You'll have to do several jobs in one day to have a decent day. You're right, the pay can be very low on a per hour basis when you're starting out. As you learn how to do each type job you'll become faster both on site and doing the report, so it will get better.
For example, I do one particular job in large numbers that pays $12.50 per job with each job requiring about ten minutes on site. I have done as many as sixteen of those in one day with most of the day spent getting from site to site. This is a job that can be reported later, so I didn't have a stack of reports I had to do that night. All in all, that was a pretty good day for the hours I put in. Not every day turns out to be below minimum wage.
I don't recommend this as a full time job, although I feel certain many forum members do this full time and do very well with it. I'm a part timer supplementing other income and the beauty of this for me is I can work when I choose and take off any time and for as long as I prefer. As you get older and older this will become more important to you.
Like your little old lady, I am in my seventies, not frail but with multiple issues that require attention and prevent my working every day. I've been doing this since late 2009 and this is the most interesting and most flexible part time work I've found. Working part time, I earn between $10,000 and $15,000 per year. That's not much money but it keeps me out of my savings and lets my modest investment income roll over into my accounts.
As long as you're able to handle a full time job, that would certainly be preferable if you don't mind working every day. When you reach the point you want to be a flexible part timer, I think you'll really like this.
Mary Davis Nowell. Based close to Fort Worth. Shopping Interstate 20 east and west, Interstate 35 north and south.