I've decided I'd like to get a set of the Trijicon 3 green dot sights for my P99. On the Trijicon website they have an msrp of $99. Looking around elsewhere I see generally $80-85. Now, I've heard that they can be somewhat difficult to install, why is that? If it's not something I can/should do myself, how much should I be expecting to pay at a gunshop? I'm gonna call the gun store tomorrow and get a quote, just wanted to get some advice going into this.