can we change the product quantity if we use the payment request API? as for now, i see example of changing the payment method, changing user contact information, changing shipping address and also selecting various type of shipping address but i don't found an example that shows buyer can change quantity of the item
for example the buyer click buy now, and then that Payment Request API interface would popup. In the popup, i want the buyer to be able to add quantity of the item he or she want to buy.
That is not possible within the Payment Request API spec. If you want to make it happen, consider proposing the feature in the spec discussion at https://github.com/w3c/payment-request/issues .