Transferring the value to the Paypal field after selecting credit card in the payment options

IF payment is by credit card, I would like the monthly membership amount to be transferred automatically to the donation amount box

You need to get the total first by calculation before passing it to the payment field. I did it for you. But here are the steps to do that:

1. Get the total. Basically, you just need to put the membership field in the calculation. 

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

2. Pass the total to the payment field, this guide: https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field. 

Let us know if you need further assistance.