Yeah, $900 for a Mossberg combo package seems way high to me. When I want to get a good feel for the "street price" of a particular gun, I usually go to both Gallery of Guns
. Both will give you a good feel for how much the gun should cost. MSRP is just that, "suggested" retail. The dealer cost is roughly 25-40% less than MSRP, and most dealers mark up 10-25% above their cost. Gunbroker.com and galleryofguns.com will give you what you will actually pay for the gun. Galley of Guns "Gun Genie" allows you to find the actual gun you want, and if it's in stock at Davidson's (which is a gun distributor and the ones who run the Galley of Guns website), you can put in your zip code and it will give you the actual prices for your local dealers and allow you to buy online (though you still have to pick it up and pay the balance at your chosen local dealer's shop). Gunbroker is like Ebay for guns, obviously minus Ebay's VERY anti-gun attitude. I'd strongly suggest checking there as well. I know that it's good to "support the local dealer", but it is possible to find some very good deals there, and frankly, I'm not going to spend a lot more money just to support a local dealer if he's ripping me off. I bought my most recent rifle and a Rock River Arms lower from dealers selling on gunbroker, and both transactions went smoothly and easily, and I got very good deals.
And, yes, it will most certainly function perfectly with any shell 3.5" and shorter, so 2.75" shells will work great. This issue is only really a concern with semi-auto shotguns, since they must be adjusted to cycle correctly with the maximum powered 3" or 3.5" shells without damaging the action, sometimes they won't cycle lower powered 2.75" shells well or at all. Pump guns are "arm powered", so they don't care what shells they feed (usually).