Be sure to find a shop that is equipped to charge by weight and not pressure alone. Most American cars use a CCOT system (cycling compressor with fixed orifice tube) and the pressures are a good indication for charge level on these. Our cars use a TXV system (thermal expansion valve) and the pressure readings are not nearly as reliable for this.

Some shops charge by weight, but not all of them.

You should be able to get the replacement mounting piece from the dealer. It would be a lot cheaper to order online if you can find the part number for it, but finding that number is not easy even with the parts catalog (which you can access for free, BTW). Getting one at the dealer is probably a good idea, but keep the package in case you need another in the future - but you probably won't.