1. You have absolutely no understanding of the size of your market, the frequency with which they buy shirts, the price they're willing to pay-- or even whether anyone needs a custom-made shirt on 48 hours.
2. You're not a tailor, you apparently haven't worked in the fashion, textile or manufacturing industry and probably couldn't tell me anything about how shirts are made.
3. "Custom-tailored" means "built to fit a specific, usually unique configuration." If "cuffs, collars and buttons would all be standard", you're not custom-tailored.
Also you don't mention fabric, which is one of the most important issues.
Also, most tailors would tell you that if you're just entering measurements in whole integers, it isn't custom-tailored. Now If I can enter "18 3/16 x 37 5/8", that's closer to custom.
But a sine qua non of custom-tailoring is being measured for the garment by someone who knows how to measure.
4. I'm going to measure myself? And there is a no-questions-asked return policy? That should be fun.
5. You're planning to get product from wherever they're made to the buyer in 48 hours? You know that most of these shirts are assembled in Asia, right?
They're all minor issues, though. Hey, if Outbox-- or grocery delivery services-- can get funded, I'm sure you can too.