In the scenario that you describe and in most ecommerce setups I think you should have an orders table to collect the high level info like the amount of the order and the date as well as the id of the user who made the order. You should also have an order details table that will have a record for each item in the order. This is where you record the item and the quantity that was placed with the order. The third table would be a users table to hold all of the information for the user like their name, shipping address, email and any other details that you might need for the user.
If you have not taken a look at the ecommerce sample db I would suggest that you check that out, it should have a structure that you could modify or use as is for this. This is available from our free tutorials and training area: