How are you advertising it? Because I'm sorry, but if commenting on videos is how you do it then it's not a very good strategy and might annoy some creators (not me, I'm in e-commerce so I understand the struggle lol).
You should send it for free to some of the bigger channels and blogs on steemit and Dtube and ask them to wear it in a video and have a link to purchase in the description. Better yet, offer them an affiliate program where they give the buyer a 5-10% discount, and they get a 5-10% commision on anyone who uses their discount code, that way they will have an incentive to have the purchase link on every post.
If you're very serious about it, you could even run Facebook ads against people who are members of different groups related to steemit.
And lastly, think about the value this shirt is giving to your customer- why would he buy it? Do you want to make him feel a part of something bigger? Then maybe add some internal steemit joke in the design, something that only people from the platform understand? In that regard sending it to Steemit influencers is a great way to turn it into a trend. I'm not really a big merch buyer so it's hard for me to think like the costumer, but it's definitely something you should think about if you're trying to sell it.
Good luck:)