Firstly, what model of MacBook do you have? If you have a retina, you will not be able to upgrade the internal flash memory, as Apple uses a proprietary connection for the flash memory. There's a possibility that 3rd party manufacturers, such as OWC, will release their own flash drives. However, none have been released so far.
Secondly, do you need the extra space? How much are you using right now, and what do you plan on doing with it? If you are simply installing Windows 7, that shouldn't require more than 40GB. You can go with even less memory, though you might start running into issues.
If you are set on replacing the SSD, you might wish to consider doing the upgrade yourself. It will save you a lot of money to sending it to Apple, it's pretty easy to do, and it's good to see what your computer is made of.