Merchandising can be a good supplement to your mystery shopping income and, unlike mystery shops, you can work and earn money steadily for most of the companies that you work with.
You can be paid hourly for recurring jobs or by the project for one time jobs.
The index at volition.com (among others) has a merchandising chat board where you can ask questions and learn about merchandising and the companies that do it and, also, a list of merchandising companies that you can register with.
Good Luck.