At the bottom of very page there is a "Contact Us" link.
As to the Premium Member Subscription plans...
Since you are already a full member (at least 50 posts and been a member for 90 or more days), there are two options you can choose from.
1) Year subscription (auto-renew at the end of one years time)
2) Monthly subscription (auto-renew at the end of one months time)
You can cancel at anytime and you will still be Premium until the purchased time expires. If you do choose to cancel, once your purchased time has ended your sales threads will be removed as you will no longer be a Premium Member.