For completeness/posterity, also added to StackOverflow:
The annoyingly tiny answer was that the first argument to the PaymentRequest constructor is supposed to be an array.
const request = new PaymentRequest([ applePayMethod ], paymentDetails);
(Or, make the variable an array from the start. The two examples I'd seen used different approaches, I missed the array brackets.)
dougbkyndryl
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(Administrator says using a gmail account instead worked fine.)