I don't like the idea of sending my clothes to some company, plus it will cause a logistics headache for you. you can just show a guide on how to take precise measurement of the user body and enter it online.
These are two problems I definitely saw coming. The trust element is a big one, and I've thought about doing what you suggested so as to avoid the logistics, but different jeans and clothing brands fit differently. I think if I have people send in clothes and perhaps have a spreadsheet of how brands fit, I would eventually have an heap of data where I wouldn't need people to send in clothes.