I know I myself hit the maximum shell count well before any of my other materials had even thought about making it there, so I'm sure that others have hoards chock full of shells - and no way to use them!  

I think it would be neat to introduce a craftable item called a Shell Trumpet, which is similar to a Bandage, or the pet ability which allows them to escape one battle per island (brain fart guys).  It displays as a small icon within the battle screen that one clicks to escape a fight without losing an energy point; the total number of your Shell Trumpets would be listed here as well, much as the number of Bandages (and when you can use one!) are clearly denoted on the explore button menu.

At this point you're going, "but that would diminish the point of that pet ability..."
These pets would now be able to flee from two battles per island, rather than just one, keeping their abilites valid!

To make the items more valuable and to keep the market from being flooded with them (if this is a concern), I imagine the schematic could be made to have a low drop rate (such as the climbing gear sets) or would require a pet with specific abilities (artisan, maybe?) or very high stats (90+).  However, I think they'd be used quickly enough by individuals that few would really make it to the market, much like bandages, which I haven't noticed being overly common.

Agree something like this is definitely needed