Hmmm, for me the cap wouldn't be enough, I'd just have to create an account, fake for twenty referals, then do this multiple times. But maybe I'm over-paranoid on this. Just that money/discount reward just for pushing people to signup seems to really start by hurting your business plan. Let's wait for other opinions on this.

You might consider posting your site with a different title, like a "Show HN : launching a new brand of t-shirts targetting programmers" or an equivalent? Because the current title seems to ask advice on how to go viral and a lot of people might not even know your project exists even if they would like it!