help

I believe that you misinterpreted the advertisement, could be a language issue.
Perhaps chess.com could also improve the display, saying you save 40% when compared to a monthly payments.
Anyhow, here is how you save 40%.
Diamond membership 14 $ /month *12 = 168 $ per year.
You paid 99$.
Your savings(percentage): (1- 99/168) * 100% = 41,071 %

And if you want to cancel your subscription, you should contact support, not post it in the forums:
http://support.chess.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=561&nav=0,71
http://support.chess.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=571&nav=0,71