Can I perform a calculation in my form to automatically sum up the total amount that needs to be paid?

I have been asking a lot of questions and EVERYONE that has been answered has been BEYOND helpful. I am new to all of this but I am a diy person and would rather learn myself and do it myself then pay someone to do it!!!!!!!Back in January, I made a basic registration form. I didn't realize how intricate and detailed I could make this form  until now and make it to where it can be a whole lot easier for the registrant and for myself when it's time to register. You may have answered this question for me before, or you may have answered this period. going throuh the forums and user guide is helpful but when it comes down to the detail questions, I am not sure which method is best for what I am asking or if it pertains to what I am trying to do.

I have my form linked to paypal. But as I am doing the group registration form right now (it's not completed), and I will be incorporating the multiple hide method. So when they press if they are registering another person , more fields will populate each time. I have also included with each group if they will be purchasing t-shirt. they have the option to select yes and the size. I noticed that with paypal, everything has to be done at the end and they have to manually input how many T-shirts they want, sizes, how many people will be paying the admission fee (we will charge for this $20 entry fee and the T-shirts will be $20). Is there a way that each time they click yes for regisering another person, it automatically totals it up and all they have to do is press submit and it takes them to paypal? Also with each registration block, if they do select that they want a T-shirt can that autmatically be calculated as well? Can everything be totaled up and the registrant won't have to do it?

I hope what I am asking makes sense and I hope I am explaining it correctly

You could use a calculation widget in your form to do this. Then you would need to pass its value to your payment tool to make the charge.

I suggest you to check the following guides:

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form
http://www.jotform.com/help/301-How-to-Assign-Calculation-Value

Let us know if this helps or you need further assistance with your form and we'll be happy to provide it.