Mountain bike fitting is a little tricky for a number of reasons.
Setting up a mountain bike is a combination of fit, but also intended riding style, intended riding terrain, and some personal preference. For example if you are doing a lot of downhill riding you are probably going to want wider bars and a shorter stem and possible handlebar a little higher (or use a riser bar). However riding XC, the set up will be different.
If you already have had mountain bikes and have experience setting them up, then I wouldn’t hesitate ordering. However if you are new to mountain biking, I definitely would go through an experienced shop and get them to help you out. If there isn’t a dealer for you locally, it may be worth taking a trip to find a good shop that can get you properly taken care of.