If a substitute is trying to click on the PDF link and it will not take them to the shopping cart or open the link for the course, it can be due to the Internet browser they are using. Try using Google Chrome or Firefox instead of Safari or Microsoft Internet Explorer or Edge and then click on the link. You may need to copy and paste the link from one of the browsers to another. This is accomplished by doing a Control C and then a Control V in the other browser window.
This is Internet Explorer:
It's highlighted by doing the Control C.
Open Google Chrome:
This is Chrome, with the link pasted (Control v)
Then the sub can get to the shopping cart and purchase the course.