Should I make premium cheaper?

I don't really know this market or any of your competitors, but $15 seems like a very low business expense to me. If I would want to quickly get my design out, or update my online store, I doubt I would really spend hours comparing solutions to save a few bucks per month. I would rather make sure I pick the service that looks like it would work well, and that would make my t-shirts look good to my customers. With such a low price point, I would focus much more on value/quality than on lowering price.

I get the feeling your Premium pricing plan has a lot of greyed out "coming soon" features because you feel unsure about your value? Maybe you just started out, and your competition has all kinds of additional features? I wouldn't worry about that. Focus on what you do well, not at what you don't have (yet). Best case people didn't care about those features, worst case it makes your product look incomplete/beta. Maybe your support is much better? Perhaps in your initial growth phase, there are plenty of "Do Things That Don't Scale" extra's your can help your customers with.. like help them import the images on the store (just an example; again, I don't know too much about this market).

Perhaps you can even experiment with more pricing plans, like a single-use $15 for just one mock-up.