Vamos a ver, ese problema lo tiene todo el mundo (yo incluido), cuando quieres pagar con Paypal a vendedores o a tiendas online de Japón con una Mastercard, pero esto no es de ahora...
Como aparece en una de las tiendas online:
"Unfortunately MasterCard's processing rules are strict regarding cardholder transactions. One of these rules states that for international merchants, card transactions must be handled by an in-country processor. Since PayPal does not currently have a processor in Japan, MasterCard-funded PayPal transactions cannot be used at this time to pay for your orders. This is a great inconvenience not only to you, but to us and to PayPal as well!
PayPal is currently negotiating with MasterCard to have these rules changed. In the meantime, you may want to use another card for your PayPal payments, if possible (such as Visa or American Express), or pay for your orders using your MasterCard directly, rather than through PayPal."
Hasta que solucionen ese problema, habrá que usar otra tarjeta de crédito o hablar con el vendedor para ver la posibilidad de que cobre el pedido directamente de la tarjeta sin pasar por Paypal.
Saludos