515 kdp Vector Designs for T-Shirts and merch

Kindle Direct Publishing (KDP) is Amazon’s book publishing platform that can be used to self-publish a book online, which readers can purchase as an eBook or print-on-demand book.

Once you create an account, you can upload your book files to Amazon KDP, publish them, and readers can purchase your eBook and instantly download it to their device or purchase your paperback copy and Amazon’s KDP platform will use print-on-demand technology to print and ship your book directly to the reader once purchased. Amazon will pay you book royalties per book sold on the KDP platform.

There's no charge to upload the file. Authors get royalties of 35% to 70% of the sale price.