You can always sign up and become a merchandiser. Then, you can state truthfully that you are a merchandiser. Your job duties can be much more than 'schlepped a boatload of stuff'. You must introduce yourself to managers and staff members, understand planograms, be responsible for property, reset product, utilize apps, conduct audits and inventories, work within given time frames, or whatever your articular assignments involve. Describing overt work distracts from the covert, confidential, and propriety aspects of work that you do as a mystery shopper.
Nature does not hurry, yet everything is accomplished. - Lao-Tzu