u/jekyll-paint

I have a client who wants to buy the full rights to a commission so they can sell it for their local brand in the form of t-shirts, prints, stickers, ect. I have always wanted to work with a brand so I’m very excited! Though I only have a basic knowledge of how copyright and licensing works. Would I be transferring the full copyright rights or licensing…? How would I write a contract? Im ok with charging an upfront one time price for full use since they are a smaller brand as opposed to royalties, but I have never done this before and want to make sure I’m not fucking them or myself over. Any advice for anyone new to this would be awesome.