If you are being redirected to the bookstore website you could be using a non-First Day link, or an older First Day link.
If the link you are selecting is titled "Purchase Course Materials" then this is not the First Day link. This is a link that will take you to the bookstore website.
If the link you are selecting is titled "Course Materials" and is taking you directly to the bookstore website, then this is the old First Day link. Check with your LMS Administrator to ensure you have added the correct link to your course.