Well you could try to tag some of your followers on the pictures to increase the comment rate. I think if someone likes a post and comment on it, it´s more possible to reach more people ( please correct me if I´m wrong, that´s my experience )
I make something similar, dropshipping tshirts on teespring but I started with posting content and build a little fanbase. as soon I hit 2k on my account I made a giveaway post with a nice design, told them to tag friends tec, I actually wanted them to engage more. And I work with direct messages, this is something you could try as well but not simply: here get my shit but be more random and don´t try to sell them in the first place. That´s how you build up a fanbase.
If someone likes ninja turtle shirts but don´t want to buy now, they will sure buy a shirt later from someone who engaged with them most. And try to be consistent with your postings. Choose a subtheme like a certain color scheme or somehing people recognize, so next time they are at a zoo and see some turtles they think of your ninja turtle on pink background.
If you want to get serious in selling stuff you should be also trying to reach your audience on other channels, like a marketing mix. IG, FB, a nice SEO Blog and than hit them with a nice E-Mail sequence. Than you can set up a selling funnel and warm your audience up.