1st Table - [item_code (PK) , details, availability, price]
2nd Table - [Cus_ID(PK), Customer_Name, mobile No, address, date ]
3rd Table - [id (PK) ,Cus_ID(FK),item_code (FK) , quantity , price ]
Use Where Cus_Id = "24" Clause to Print Invoice --- And I am not Familiar with .net ....