Found something: crabs can be led to fly inside the shop by being on the far left when they go left, and are lined up with the shop.
Thanks so much! I'm glad you've been enjoying it! I'm not currently planning anything with this overall concept, but I might revisit it at some point (I hadn't planned on expanding on the first game initially either).
I will probably have to set aside some time to do some bug fixes though! it sounds like a few things fell through the cracks - I don't remember off the top of my head what the upgraded thopter was meant to be called, but I might have it in my notes somewhere (and forgot to put it in the game).
As for difficulty, it "stops" once all available spots are filled and upgraded, but that would be pretty late (and difficult) - I think like level 176 if my math is right?