I recently swapped out 2 of the under-performing Little Trees selections with Leather and Blackberry Clove. They both do much better than the ones replaced. (Coconut and Vanilla Pride)....Black Ice is still the number one seller.
I agree on trying various things to work in your area. And about the Sams/Costco microfiber towels. And Black Ice air freshener.
btw the Shurvend at my wash is "old reliable." Sells and sells, almost never needs attention. I won't get rich off its proceeds but for little work it is easy profits. I wish all the rest of the equipment matched that. Every time I restock the vendor I give away trees or towels to people washing cars. I just tell them I had a few extras and thank you for using the wash. So that builds goodwill plus maybe incents some repeat business to buy more vending products in the future.