E-junkie cart buttons should work fine in an email; they would display the cart in a separate browser window/tab. If you would still prefer to use Buy Now buttons, which bypass the cart and take buyers directly to instant checkout for just one item at a time, but you are finding the tab for Buy Now button codes is greyed-out, that indicates you have configured the product using Variants/Variations or "Let buyer edit price", which cannot work with Buy Now buttons.