Be aware also that it can be sometimes that the size of the seat post is too small for the seat tube. I was upgrading an old cheap MTB with 26.0 steel seat post with the nutted collar to an alloy seatpost with single 6mm allen bolt. To my surprise, the old metal seatpost, even though it was stamped 26.0 measured more like 26.10 and and the steel one would occasionally slip when testing it out. The 26.0 alloy post was clearly too small. I managed to have a 26.2 in stock and found it a tight fit. So I borrowed a neighbour's reamer to just clean out the seat tube a tiny bit, and managed to get 26.2 seatpost to slide in nicely. Now it bolts up fine and no slipping. But it may be good to have the shop check also for diameter of the seat tube to validate the size of the seat post is optimal.