Connecting Books, T-shirts and a Product

I need help and advice to help me implement my ideas: I have a Simple Site webpage where I sell two books. [login to view URL] Vibrant in Menopause And Luke’s Rescue by the Bach Flowers-a children’s book about Dr Bach’s Rescue Remedy. I have recently self published on Amazon after getting the files from the previous publishers. From my book Luke’s Rescue by the Bach Flowers I created characters out of the flowers they represent and I would like to sell t shirts with the characters printed on them. I have a Printful (t shirt fulfilment website) account that can fulfil any orders but I need to link it with my website or an Amazon account. I want to connect with Printful perhaps with Amazon because I have to do the best thing for marketing purposes. I would also like to wholesale Rescue Remedy with my book and T shirts. I don’t know how to put everything together and make it easy to market. Can anyone help me and How much would it cost? I would appreciate some help. Thank you.