Thanks for the tip off here!
We seem to have an issue with the shopping cart from time to time where the discounts don't appear the way they're meant to. Sometimes this is because there are other items in the shopping cart, or because there might be some sort of record of a previous purchase attached to your system.
It's a little bit hard to troubleshoot exactly why it's happening to you, but if you hit 'clear cart' and try the link, you should find that the course shows up free.
99% of the time that works (and I think it should, since the discount works okay from my end), but if you're still having issues just give us a buzz and I'll take a closer look. If worst comes to worst, I'll always be able to refund you if anything goes wrong
Let us know how you get on!