"she was asked to buy like 70-80 items at a grocery store"
I think you have been misinformed because I haven't heard about such a demanding shop. I have done a grocery store mystery shop audit where you get the prices for about 50 items (maybe more, it has been a while).
As for grocery stores, the three that I am familiar with offhand require 1) a minimum $10.50 purchase (nothing specific), 2) A purchase of some kind (nothing specific), and 3) Two required items - A sliced deli item 1/4 pound or more (sliced to order) and a bottom of the basket item (something like a case of water, large bag of pet food) which can be later returned at your option.
As for clothing, the ones I'm familiar with typically reimburse a small amount ($10-20) on a purchase that is not specified. They just want you to evaluate the checkout process and, of course, provide proof that the shopper was there at the time of the shop.
Happily shopping Rhode Island and nearby Massachusetts and Connecticut