This may help others... You say "not sure why people are having problems..." That's because you need to open the cart to apply the code rather than going straight to checkout. When you click on the cart icon you have two options 1. open the cart, 2. go to checkout. You need to open the cart. It's not obvious.
I don't know what the code or process is - I just had my auto-renewal of VIP membership, and last year only ordered a sparesafe (presumably at full price) but now need to crack on with a topper etc.
Would you be kind enough to PM it to me, or post a link as to where others will find it? - I doubt I'm the only one wondering.