[w3c/payment-request] Allow default option for donation in PaymentShippingType (#638)

If this API is used for accepting donations (for example, a 501.3c organization is the US), the default delivery options do not make sense as no goods are being exchanged. It is not clear if we can avoid using the default `PaymentShippingType` ENUM and replace it with a custom option (instead of adding to the end of the default list).

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