The hazmat is collected by the seller to cover their administrative costs - there's a lot of paperwork involved. Notice the price isn't the same everywhere - Cabelas charges $20 instead of the usual $28.50 most places do.
I have no idea why you might get different results with cannon fuse.
Sending caps via USPS was probably massively illegal, that's the answer to that part, so you got the don't ask don't tell discount.
Personally when I last ordered musket caps they required a 2K minimum order. I've had lots of offers to buy a tin here and there since they are pretty much impossible to find locally. And I went the other direction - I put musket caps on my Howdah which is not that reliable otherwise. So that's two caps each time I load it up. I still have a ton though. But I've literally never seen them locally and I have two long guns and the Howdah that use them so I'd rather have a ton then be hopeing somebody at the range has an extra tin.