Starting with Chrome 57 the the allowpaymentrequest attribute on an iFrame will allow the PaymentRequestAPI to function inside an iFrame. Using the PaymentRequestAPI inside an iFrame would allow for single page eCommerce AMPs.

Currently the allowpaymentrequest attribute is not allowed on amp-iframe. This attribute should be allowed on amp-iframe and passed to the nested iFrame.

Note from @aghassemi
-We would need to also set allowpaymentrequest on the viewer iframe's that embed AMP pages as allowpaymentrequest on a nested iframe will not be honored if parent iframe does not have it.
-Not sure about WebViews and whether payments are allowed by default or need to be configured.