Performing calculation on form elements

I’m using CF7 for members of our sailing club to renew their membership. The problem I have is there are different types of membership, with different fees, and then various discounts as well.

What I would like to be able to do is capture the details of the membership they require and discounts they are eligible for on the form and then calculate their fee to include in the email I generate back to them.

I know how to do the calculation with PHP and if I was using a native HTML form I would be able to do the calculation as part of the submit and then include the answer in the email I generate.

Wordpress won’t allow me to embed php, but I have managed to use a Shortcode on the page, with the relevant code in the theme’s functions.php. The missing piece is I can’t see a way to link the fields returned from the CF7 form to the shortcode.

Simple example: the Adult fee is $110. If the user has clicked the “Under 25” checkbox, subtract $50 from the fee.

The page I need help with: [log in to see the link]