They're Chinese knockoffs (there's hundreds of these sites), but they still look great.
Go on aliexpress.com (China's amazon) if you're worried about inputing your credit card information to that specific site you listed, and you can browse various bootleg shirt makers and read their reviews. Honestly, if you're in the market for a jersey, you'd be crazy not to buy a Chinese knockoff. I've gotten several from various different places, including a Brady Michigan jersey that looks awesome. They're all stitched and embroidered and have patches, just like the on field ones that you'd pay $300 for. 9/10 you can't tell the difference with the official on-field ones, and even if you can notice some small differences (sometimes the patriots logo right under the collar can look little funky), they still look FAR better than the silk-screen ones you'd get for $100. Plus, you're not giving money to the NFL.
I was actually going to buy a Malcom Butler one this afternoon. Here's a link for someone's review of bootleg NBA jerseys they bought
https://www.reddit.com/r/nba/comments/17vayb/took_a_risk_buying_jerseys_from_a_china_wholesale/