I love my Ikea mattress as well! It's been 3 years and going strong. Mattress preference greatly varies so I certainly would not discourage buying from Ikea, but you should think about in case you didn't like it.
As poppydog mentioned, it is my understanding that Ikea's delivery fee structure is different between online order and in-store purchase + delivery. I believe latter is cheaper. Yes, you have to drive to Charlotte, but if you really want to save, it might be worthwhile. I'd give the Charlotte store a call and inquire. They may relax the zip code border thing?
IKEA Charlotte Services - IKEA
Or, you can wait until they come to the Triangle
Finally, I vouch for NC Modern Furniture. I bought apartment load of furniture so it was well worth it for me. They provide really great service.