Bypass membership level selection

My site has one free membership level. Membership is by invite only and entered manually. I want a hidden (not in a menu) registration form with invite code, but don't want users to have to select a membership level... etc. At present they have to 'Select a subscription', there is only one! How can I change that? Also how can the form font be changed to fit the site?
http://kyebay.ca/register-2/
Please advise.