Integrating Zip into other ecommerce platforms is straightforward with the documentation.
After you add the Zip payment gateway and integrate the Zip widget, be sure to check out the Testing & QA Guide.
You should not reuse checkout session id when starting orders. A new session id should be created each time.
Use case #1 - If a customer at checkout decides to go back and edit their cart, a new session should be created. They should not reuse the same checkout link.
Use case #2 - If a customer has modified their checkout between navigating away (canceling) and coming back (starting same checkout) that difference won't be shown in our system if the checkout link is reused.
Updated about a month ago