Hey,
before creating user stories you could have business process modelled with Task Descriptions. They will define what people are doing on that eCommerce site. BA should create those Task Descriptions and give it to UX designers so they can design wireframes. They should make sure single Task Description is covered on one page of UI without any navigation.
To read more check Soren Lauesen article:
https://www.itu.dk/~slauesen/Papers/IEEEtasks.pdf
User Stories are rather execution than modelling tool.
Good luck,
Lukasz Pasek
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