Dump the grip shifters entirely and go with trigger shifters like these Shimano Tourney version. I have used this style shifter in either 3 x 6 or 3 x 7 type for years. Cheap and effective.
Stupid for shop to switch you from one grip shifter to another.
https://www.amazon.com/Shimano-Tourn...er+3+x+6&psc=1