How do I pay Paypal for a purchase they paid for ??

I'm not sure that's how paypal works.

Typically, what happens with paypal, is you sign up for an account (which you may have done)

After you sign up for an account you login and add in your own payment information, usually a credit card that you have, and your address and phone number etc.

Then when you purchase something and use paypal as the payment method it will charge the card of your choosing and pay the vendor. By signing in to paypal after your purchase, you are authorizing them to bill your credit card. Then the payment for the item should show up on your credit card bill.

So make sure you have edited your payment methods in paypal and you should be OK.