Multiple values for unique id?
I would create three tables instead:

Customer table:


Customer_id, name, address, etc. 
PK: customer_id


Boxes table:


Box_id, size, description, etc. 
PK: box_id


Orders table:


Customer_id, Box_id, Quantity, Timestamp. 
PK: all four fields.
Foreign keys box_id and customer_id to the respective tables.


You will have one entry for each customer, each box type, and as many boxes of different sizes ordered as you wish.