JavaScript - Js And Formmail

Hi, Sorry very beginner in JS
I am using formmail and JS to collect booking info calculates a total and sends to email. Formmail redirects to a thankyou page. Here I can send the total to paypal. I have looked and looked, but cant work out how to pass the total from the booking form to the thankyou page. How can I do this simply.
Code not supplied because its nothing wonderous.
Thanks