registration form with extra calculation

Hello wonderful people,

This is my first post asking for help on a matter:

I have a site where people will register for an event. I want the to be able to pay in paypal, but BEFORE they need to input different data such as if they have a local tax number choice 1 input the tax, indication if it is the wrong format.

so basically it is a registration form with a calculator for Paypal. I only want them to go to paypal having to pay the price for one item.

I use contact forms 7 which i read is not the best. If I pay I might get 123 or Gravity but for the time being the event and company have not got enough “juice” for me to be able to do that.

any suggestions?

thank you for your answers!