The primary benefit of Premium Membership is getting discounts on the premium ebook credits.
Free trials are only for normal memberships, so if people buy credits during them they pay full retail price.
It's not really my intention to lock people out of getting the premium ebooks and bonus stuff unless they are actively subscribing to premium, as everyone has their own financial situation. The free trial loophole is fine. If you think that's unfair to you as a premium member, remember that they're paying more for those credits and not able to read any weekly releases.
I've thought of a few other perks for premium subscribers in the future... maybe like a custom member's card for people that've been premium for a full year, or something.
But trying to make a digital product like short stories or bonus illustrations a "limited" item is a fool's errand and would only encourage their piracy and lower our sales overall.
Eventually I believe we will implement a full store, and in that store perhaps the premium editions will be slightly higher priced. But it's not something I'm going to focus on too soon. Practically Premium Credit sales are still only <5% of total ebook sales. Right now focusing on increasing subscribers and overall name recognition is more effective use of resources.