On the product settings page, you can override the default shipping rate for the product type.
Go to Products > Choose the item. Go to the Shipping section, and Check "Override default shipping weights"
Enter the weight and dimension for each item of this product size.
If you enter the largest number you would receive an order for, such as 10, it will charge that amount for any order up to 10. For example if you have up to 5 items entered and have completed the dimensions for up to 5, and then next line is 10 items, if someone orders 6 items it will add the shipping based on 10 items dimensions. If the order is for 11 items or more, the order will display "Due to the size of your order, shipping will be calculated separately." You can then add shipping by using the Customize Order button on the active order. The customer will receive an email notification of the additional shipping amount when you add the shipping fee.
Flat rate shipping for a particular product
You can set a flat shipping rate for a particular item. For example, if you create an item that is a Mixed case, you can set the shipping to be a flat rate of $20. When someone purchases it, $20 will be applied as the shipping rate. If other items are added to the order, it will calculate the shipping cost for the regular items PLUS the flat rate shipping cost for the case at $20.