Your customer is probably using the "Send Money" process and selecting "Quasi-Cash" as the payment type. If you sent your customer an invoice, you may have selected "Quasi-Cash" as the payment type.
To help your customer complete the payment, you can either use "Request Money" to send an invoice and choose the appropriate type of payment, or you could create a Buy Now button from our Merchant Tools section.
From the PayPal website (found by clicking the '?' icon next to the "Category of Purchase" option):
Quasi-Cash: The transmission of money not involving an underlying service or good. The bank that issued your credit card may treat this 'Quasi-Cash' transaction as a cash advance and charge you cash advance fees. PayPal has no control over these fees. If you select 'Quasi-Cash' you may want to use a payment method other than Credit Card (Instant Transfer or eCheck) to avoid potential fees.