I think the OP means B&M as he said -- Brick & Mortar
As for BTOs, there are upgrades that they will do though RAM will cost you even more (as they don't buy back the RAM that you have replaced).
If I recall correctly, during the days of CD MacBooks, there came a time when they offered a couple of configurations beyond the three basic setups on the webpage (more RAM and maybe a bigger drive), but generally BTO must be done through the website.