How to perform calculation inside of the form?

Can I add a sum, using conditional logic, that will add and/or substract amounts based on the choices made by the person filling?

I would like to take multiple fields into account but when I try it, only one of the conditions is taken into account and the result is not a sum taking all choices made into account.

If I understood you correctly you want to perform calculation inside of your form, based on options that are selected by your users. 

You can use Form Calculation widget to perform calculation in the form:

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form 

Considering that you are using radio button fields, you can check this guide to find out how to assign Calculation Value: http://www.jotform.com/help/301-How-to-Assign-Calculation-Value 

Calculation values can be assigned to radio button, check box and dropdown fields.

Hope this will help. Let us know if you need further assistance.