System Design question asked in interview

how could I

1. design a shopping cart system
   How is tha cart updated when the application is open in multiple pages
   What if the shopping cart service is down, how would you handle it
   What if there are 10M requests at a time ?

2. how could I design a checkout system
   How to integrate payment options? how to explain this in system design questions

   Any suggestions are really appreciated

   Thanks