I purchased my 2.13GHz/4GB RAM/256GB SSD 13" config from the Apple Store, walked in, bought it and left in about 2 and a half minutes flat.
What they will not sell you is a MacBook Pro with 8GB of RAM or an iMac with anymore than 4GB of RAM, and they won't sell you a MacBook Pro or an iMac with a solid-state drive. They also only carry the lowest base configuration Mac Pro tower at the store (the 2.8GHz quad). If you want a 6-core, 8-core, or 12-core Mac Pro you have to order online.
Also, on the new iMacs if you want the 2GB graphics option you have to order online, they don't carry any of the upgraded models in stores.