CoreCommerce allows you to customize shipping rates based on order quantity. To configure quantity-based shipping:

• Log in to the admin panel of your online store.
• Click on the gear icon upper right-hand corner →  Shipping settings
• Click on Setup Shipping Zones > EDIT the Shipping Group you would like this method in. 
• Select “Order Quantity” →  click Configure
• Now you will need to name your Method. Typically the name indicates the Carrier’s product title. e.g. “Priority” or “First-Class”. 

• This menu allows you to input multiple Rates. Complete cost for first item and additional items. 
• Click Save