how big will be the change in my code to store that data from the shopping cart.

Really depends on how you implemented your code now and how you implement the new code to store. If you are storing to a DB then you will have to make some api calls to store the data in the db and retrieve what is stored in the data. If you were using one of the client storage options you would either make a call to the api or add it accordingly to the client side call to do that