How can I caculate the total price of the selected products?

You will need to first assign calculation values to your options and then perform the calculations, once you have the total amount you can pass it to the payment field. 

These guides will help you with the process: 

https://www.jotform.com/help/301-How-to-Assign-Calculation-Value

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

I hope this helps.