Help with a multiple choice payment button

First, I was not even sure where to post this but this forum seemed logical.

I have a membership site with several levels of membership, each at different price points.

I am offering an optional continuity membership with each fixed term membership and I need to figure out how to set up the payment button. I am using Authorize.net. The continuity program is the same for each level of membership

I want the customer to fill in the required billing information for the fixed term membership and then I want to display a pre-checked checkbox to indicate acceptance of the monthly continuity program. If the customer accepts the continuity program the purchase needs to be recorded for the fixed term membership price and then continue billing monthly for the continuity after a 30-day free trial.

If the customer un-checks the checkbox to decline the continuity offer he is then billed the one-time fixed term membership price only.

So, can anyone suggest a method or point out a working page that I could edit to my purposes?

Anything you can offer will really be appreciated.

Thank you.