Hey, The system expects buyers to log in to PayPal and choose that credit card as their funding source if they want to pay with it.
As for testing on the sandbox (other than PayPal's credit card generator), I've always used old, expired credit cards or [URL Removed] (Third-party site) and they seem to work fine for me. Always remember you cannot purchase anything using fake card details. Credit card generator is only used for Sandbox Environment.
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