I've been using G-Mart as my comic supplier since 1999 or so, which is somewhat remarkable because I haven't lived in Illinois since 2007. The storefront itself is an excellent comic store, and I have long enjoyed stopping in to chat with the staff; but its pre-order system is even more remarkable. Yes, you have to pre-pay, but you get 35% off for your troubles, and when you've been doing it for this long it just feels like you're keeping the pump primed. And given that they offer free shipping on sufficiently large orders, I have pretty much no reason to change comic stores any time soon.
I recommend G-Mart to both Champaign residents and to anybody that's looking for a simple way to mail-order comics.