Some options you may want to consider are taller walls for added shelving and storage space, and bigger/taller doors so you don't have to duck every time you go in and out of the shed, and you can get your lawn and garden equipment out easier.
If it comes with Gable vents, get the biggest ones they have on the ends and a Ridge vent all along the peak of the roof if this is not standard, to help add in venting the shed.
Add in a few extra windows for light and make sure they have screens in them so you can open and shut the windows for added venting/cooling.
Add pressure treated plywood for the floor, as well as pressure treated 2x4's for the floor framing.
Get 16 inch spacing for the framing if it's offered for a stronger built building.
Get a longer/larger ramp for the ease of getting things in and out of the shed.
Consider a Salt Box or Carriage style shed instead of the barn style. It will give you more/higher wall space. Plus it can look more like a mini version of your own house if you match the paint to your house. And it won't be so tall that it looks weird in your back yard.
My shed was built by the Amish in Lancaster PA. I was able to go up to their shop and actually sit down with the people and go over everything I wanted for my shed. This is a lot better at making sure you get exactly what you want, versus trying to convey what you want via e-mails, phone calls, and ordering options from their website.
It also gives you the chance to actually see their sheds in person, as they always have either sample sheds sitting around or other customers sheds there waiting to be delivered!
Most of all, have FUN with it!!!! They have so many options available its mind blowing!!! You can get exactly what you want, and like you said, it will probably cost you less than if you did it yourself, and you will have it a lot quicker!
Jim