Attaching crank arm with Shimano SLX M660
The cap is like the star-fangled nut on a threadless headset - it's only there to hold the crank in place when you do up the pinch bolts.
If the pinch bolts aren't holding the crank in place on their own, something else is wrong, and you really need to fix that. If the crank splines are worn because the shop fitted it wrong in the first place, they should replace the crank.