JPEG is lossily compressed and only 8bit colour, and while 4.8MB is still pretty decent for a 12MP image, it's still likely to be discarding a lot of data compared to the raw. If you really want high quality output from raw originals, you probably want a format like PSD (Photoshop) or TIFF that supports 24/32 bit colour with lossless compression. Most print labs would support one or both of these, although they may have some restrictions on the specifics of the file formats - especially for TIFF.
For photographic prints, I'd also look into more specialist outlets that target the more serious end of the photographic market - regular camera/photography clubs and forums should be able to provide some local recommendations for this, but mostly you'll upload images via the web and get your prints in the post, so don't limit yourself to outlets in Wisconsin. Snapfish and Shutterfly seem fairly well regarded in the mail order camp, MPix is a more professional lab that's worth checking out as well.
That said, finding a local print lab may be a better bet though. It can be quite bewildering with a large range of paper types (glossy, matte, silk, lustre, ...) at various weights and textures, plus other media (acryllic, canvas, and metal are all fairly common) available, and picking the right options can be confusing. A good lab will be able to discuss your requirements and suggest what media will work best for you once they see the image. It's also a LOT easier if you can call in and look at some samples, although some outlets may also be able to send you a sample swatch through the post to help you decide. Alternatively, you may need to do some postcard sized prints of the same print on various likely materials to find the one that'll work best for your specific image(s). If in doubt, glossy and matte paper or canvas are probably your best and safest options for landscapes, at least until you get a better feel for what the various options are.