FYI (email received from C&S team)

I can certainly provide some information in regards to the new flat rate Express post satchels.

It may help to know that when processing your item, once the product code is entered, when you select 'Choose Available Services" the relevant flat rate postage price will display for both Parcel post Plus and Express Post for you to select. Although there is no provision for you to use the flat rate Express Post satchels at present, we are happy to advise this option that will be available very shortly in Click and Send. 

Any updates and new features with Click and Send will be advised to the public via the welcome page. I hope my email has answered your enquiry and concerns.