Not sure why penguin hotlinked ar15.com. I can't figure out how that helped answer the question you had here, but I'll try.
The original price you see listed is for a regular run of the mill AR15 upper (gas tube/direct impingement).
If you want to upgrade to a gas piston, the drop down menu will handle that. If you want a piston, my personal preference would be some form of pinned piston block to eliminate any movement compared to a clamp on style.
If you prefer to have a certain handguard over another, there are several options that are compatible with the piston. Not all handguards will accept a piston. That's why there is different drop down menu's.
If you get a gas piston, leave the front sight/gas block option blank. If you didn't want a piston and wanted a regular upper, then you would pick whatever gas block you liked.
If you're allowed to have threaded barrels and flash hiders by state/local law, pick the one you like.
For those that are new to AR's, I can see how that order form could be confusing not knowing what works and what don't with each other.
Of course, you could just call them and I'm sure they would walk you through the order so you get exactly what you want.