What happens if you just try and buy it 'normally' from the FM site, as opposed to following the link?
By the way, if you like FMGo so much that you'd like to buy it again, you can't. Once you've bought FMGo once, you can't buy it again. (Nope: me neither. I know this because I introduced a customer to FM by using that offer. Another customer wanted to use it to expand their network, so they asked me to buy it for them. iTunes wouldn't let me buy FMGo again, so I couldn't buy the discounted FM Client. Apple and FM each said the other had to sort it out, but in the end they agreed that it was impossible.)