I had same problem when I tried to upgrade using the Safari on iPad. I would suggest try to upgrade on a different tool (phone, computer, iPad, etc). Once I have paid for the course, I had no problem accessing the materials on iPad, iPhone, or Mac.
All of his LR lessons use questions from before PT36 as practice with explanation videos, and there are more practice sets with each package upgrade... so if you're looking for some, thats where you can find them :D
I agree w/ Dillon PG, I think the *easiest* way to increase your mark right now for December 6 would be to make sure you're perfect on LG. Know them inside and out!
Oops, I realized I never actually answered your question!
If you go to the "Course" tab towards the top, within the drop-down menu it should say "Extensions and Addons". There you'll be able to purchase preptests and extend your access to 7sage.