You may try to pay and encounter the error message: “We can’t process your payment right now, so please try again later” after entering all their credit card details in the PayPal interface. You also might be getting looped back to the beginning of the payment process over and over again.
The value of the transaction has pushed your credit card over the “Guest Checkout” transaction limit -OR- your credit card/email address is associated with an existing PayPal account.
You must sign into a PayPal account to pay the invoice - you can still use a credit card once they are logged in.
PAYPAL LIMITS AND SOLUTIONS
1. When using PayPal Standard (you don't have paypal account), a credit card has a total lifetime transaction limit of approximately $4000. This value is the total of all payments ever made through the PayPal Guest Checkout Experience on that credit card. Even if the total is below $4000, you can only use that card 15 times as a guest. If the card is flagged by PayPal, an account (username/password) will be required for security reasons. In either case, you will need to sign up for a PayPal account in order to use that credit card.
2. If you have an existing PayPal account and the credit card or email address is recognized, you MUST log in to PayPal first for security reasons.