Can I ask where you got your molds? Do you make them? For a long time I've been wanting to make something like the old fashioned clear toy candies, but the molds are typically antiques and too expensive for me to want to buy just to play with. Are these single cast pieces? And nice details, seriously!
When it comes to sugar, theres not really a preservative in that sense. Like you said, you can use isomalt to get the results you want, but that is pricy. Otherwise, sugar does absorb moisture, thats the nature of it, its hygroscopic. Im assuming you already have your candy formula down, but obviously you would want to include a glucose syrup in it to prevent crystallization. And in your formula, you wouldn't want to have an acid present (such as tartaric acid or cream of tartar), since an acid will make it more hygroscopic, and you dont need it since the pieces are just being cast, and not pulled.
I think alot is going to come down to how you package the pieces. The next time your at the store take a look at how hard candy is packaged. They really are susceptible to the same problem your having, but they are packaged carefully. Hard candy is usually individually wrapped or enveloped, something like lifesavers or similar. Candy canes are also wrapped, but they are pretty much shrink wrapped, not really leaving any open air inside the package, and additionally, while its wrapped, you can easily admire the beauty of the piece. I was thinking this might be a option that can help you, along the lines of a shrink wrap (and hopefully the little arms of the t-rex hold up). Otherwise, you can pack the item in an air tight box with a desiccant, and again hope that during shipping the little delicate pieces dont snap.
How big are your pieces? Its sort of hard to tell from the pictures. Also, just wanted to say again, your work is totally awesome, I havent found anyone that makes this detailed of cast edible pieces. When I've searched for clear toys, some look very cool and detailed, but are made with molds you cant really get anymore. Anywho, keep up the good work!