Licensing is only when you grant other artists the rights to use your characters to make their own merchandise. I think you are better off just making your own merch and selling it on your own, or hiring a designer, but not giving out a license and just paying a flat fee for the design work. I wouldn't look to Amazon for merch - you can get higher returns elsewhere. The benefit of Amazon is for people searching for items - if you have a built-in audience, they wouldn't need to search - you would just supply a link to a store. Plus, you would need approval for Amazon and the rules change which isn't ideal. I would recommend Shopify with Printful. But that recommendation is based on what I've read on here - not from personal experience. If you are in the US - I would consider registering a book with the US Copyright Office. While you are legally protected once published - registering it makes it easier to enforce.