It'd be great for there to be a standardized way for developers/merchants to provide data back to the PaymentRequest after it's been accepted. This would allow additional information to be provided back to the underlying platform payment method handler/UI.

I was thinking it'd be something like this:

partial interface PaymentResponse {
  [NewObject] Promise<undefined> complete(optional PaymentComplete result = "unknown", optional PaymentMethodData paymentMethod);
}

or maybe even just

partial interface PaymentResponse {
  [NewObject] Promise<undefined> complete(optional PaymentComplete result = "unknown", optional object paymentMethodData);
}

Alternatively, we maybe might want to wrap the PaymentComplete in a new IDL dictionary (just like PaymentValidationErrors), which would also allow for more extension in the future if desired.

enum PaymentResultStatus {
  "fail",
  "success",
  "unknown"
}

dictionary PaymentResult {
    PaymentResultStatus status = "unknown";
    PaymentMethodData paymentMethod;
}

partial interface PaymentResponse {
  [NewObject] Promise<undefined> complete(optional PaymentResultStatus = "unknown"); // Would we still want/need this?
  [NewObject] Promise<undefined> complete(optional PaymentResult);
}

Though (in both cases) I'm not sure if we want just a PaymentMethodData or a sequence<PaymentMethodData> so that a developer/merchant can declaratively provide data back to all supported payment methods at once instead of having to adjust the data based on which payment method is currently being used.