Joomla,Proforms和有条件的Paypal

I am rather new to Joomla, and have been asked to add a form with a PayPal option. So, the requirement is to redirect to PayPal on submission of the form ONLY if a checkbox is selected.

Is there a documented way of doing this with Proforms? I can't seem to find a way to do it. Also, I will need to change the cost of the purchase depending on a specific code the user will be entering in a field. Though, that bit will probably become obvious once the first requirement is satisfied.

I can easily do this hand coded, but Joomla is beyond me to be honest.