Let's run through a quick example to get a closer idea of what I’m saying.
Traditional drop shipping initially restricts your design capability because you have to use their free design software. Which means you create a mockup that doesn't really stand out.
A men’s premium t-shirt that is printed with your design will typically cost £14.99 - then you need to add shipping which is typically an 8 days lead time and costs £2.99. This gives you a total cost of £17.98 for one t-shirt.
You will need to advertise this t-shirt to try and get it sold, which means you have to add even more margin to try and cover your advertising costs.
Unfortunately you can't add too much margin because you will price yourself out of the market and make your t-shirt impossible to sell against your competition.