Transferring value from a main form to a second form embedded in an iFrame (PayPal)

I want to offer two payment methods but can only use one per form. Your resources lead me to use an iFrame for the second payment method (PayPal).

Please refer to the last page of this form https://form.jotform.co/80098706084865

I want the Total Payable value from the first form to transfer to the amount payable in PayPal in the iFrame. 

I can see this has been addressed before by cutting and pasting code but I can't follow the guides. I have tried using the guides detailing pre-populating fields, re-directing URLs, field manager widget.  

Can someone give me the 'dumbies' guide please.

I did check your form and found that you have embedded a second form within the first form. Unfortunately it is not possible to pass the value of one form field to another form embedded within the same form.

The only possibility it so use the thank you of first form to populate the field of second form field through URL parameter. You may like to take a look at the following guide which should help you:   https://www.jotform.com/help/270-How-to-Pass-a-Form-s-Calculated-Value-to-Another-Form-s-Payment-Total-Field 

Thank you. After receiving your advise and speaking to PayPal I have made some changes that will work. 

Very grateful