PaymentDetailsBase - Web APIs
the paymentdetailsbase dictionary is a mixin used by the paymentdetailsinit and paymentdetailsupdate dictionaries.
... paymentdetailsinit provides payment information when calling the paymentrequest() constructor.

Index - Web APIs
2916 paymentdetailsbase api, dictionary, experimental, interface, mixin, paymentdetailsbase, payments, reference, web payments, web payments api the paymentdetailsbase dictionary is a mixin used by the paymentdetailsinit and paymentdetailsupdate dictionaries.

PaymentRequest.prototype.id - Web APIs
when constructing an instance of the paymentrequest, you are able to supply an custom id via paymentdetailsinit dictionary's id member.