Buying My First Mac: SSD Upgrade Question

Tomorrow is tax free weekend and I'm planning to buy my very first Mac. I'll skip all the details as to why I am doing so, but I would like to know what the process is and what to ask for and such.

I've pretty much decided on the MacBook Pro 13" Core i7. With education pricing, that comes in at under $1400. The 128 GB SSD upgrade is only $90, so I'm hoping to also get that. My question is, how does Apple go about doing SSD upgrades? Should I call ahead and will my upgraded Mac be available in an hour or so, or do they ship a pre-configured one from the factory? I'd rather not wait and would really love to take advantage of the tax free weekend, so if it takes longer than a couple days I'll just get the 750 GB HDD and possibly upgrade it myself in the future.

Note: I plan to buy it at a physical Apple Store, since I don't think tax free weekend applies to online purchases.

Update: SSD needs to be pre-configured in factory and then shipped to the store, so I decided not to wait and just get it with an HDD. The 8 GB RAM definitely makes up for it, but there are times when I wished that I had an SSD (especially on bootup). I definitely plan on swapping in an SSD myself when I can get more space fairly cheap.