Hi Jeremy
Yes, thats what the credits are :)
You could click on Help & support and the bottom of this page, then on Contact Us and opt for Billing & premium membership to discuss any possible refund.
For security reasons we can't discuss your personal payments in a public forum 
Hey guys,
I recently signed up for the 7-10 day (I forget which) free trial Diamond membership, which I forgot to cancel. I want to stay a member, but I want to be a platinum ($50) member, not a diamond (I use the tactics trainer a ton, but not the chess mentor or the other diamond benefits). My card already got hit for the $100 odd bucks, so I was hoping I could get a refund for half of that and stay on as a platinum member. Is that possible?
Note: When I go to the membership page, it tells me I have ~$96 membership credits. Does this simply mean I have enough "credits" to be platinum for two years, diamond for one, or gold for 3+? If this is how it has to be, that's fine, but I would really like to only pay for one year of platinum.
Jeremy