OK, I just tested this in the Amazon Appstore and I see the same thing you describe. I purchased the Tech Doubler with actual money last September using the same Amazon account that I'm using now, and I see the same thing you're seeing.

I haven't yet tried to actually follow through and complete the purchase, but I'll do that later and let you know what happens. If Amazon's app store is anything like Apple's iTunes store, it will offer to let me download it again for free. I have some pressing work to get done soon, but I will check this out and let you know what I find out.