Hi @Silvi ,
A simpler way is to create a collection in your apps, whenever the user adds shopping content, save the user's email and item information to the collection, so that when the user checks, you can use filters to filter all records in the same email to achieve the effect of settling all purchases by the current user.
Of course, you can also use your original method, first you need to create an SPlist, then connect it to your apps, and whenever the user adds a shopping cart item, use the Patch function to save the item information to SPlist.
Patch(SPListName,Defaults(SPListName),{ColumnName1:Value1 ,ColumnName2:Value2 ,ColumnName3: Value3,....})

Hope this helps you!
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Best regards,
Rimmon Li