Questions like this are never simple Yes or No answers. Here are some of the things I would consider:
If you start and prove yourself as a Jr. Merchandiser, it seems like there must be a better paying Sr. Merchandiser position to move up to.
If you are working for several companies, picking up an extra job while you are already at a location doesn't seem like you are losing anything to take it. Going out of your way would not be worth it,
The companies I work for let us claim the contracted time for the job if we complete it successfully. While we don't get raises unless we get promoted, you can do the job faster the more experienced you get. That translates into a higher rate per hour.
Former mystery shopper, current merchandiser.