EZ Inventory has 3 types of inventory updates you can choose from to update the available quantity in Shopify:

Set Available Qty

This will overwrite the inventory quantity in Shopify with the quantity from the inventory feed.

Shopify Available Quantity = Inventory Feed Quantity

Increase Available Qty

This will add the inventory feed quantity to the current inventory quantity in Shopify.

Shopify Available Quantity = Current Shopify Available Quantity + Inventory Feed Quantity

Decrease Available Qty

This will subtract the inventory feed quantity from the current inventory quantity in Shopify.

Shopify Available Quantity = Current Shopify Quantity - Inventory Feed Quantity

Set On Hand Qty

This will overwrite the inventory quantity in Shopify with the quantity from the inventory feed.

Shopify On Hand Quantity = Inventory Feed Quantity

Notes:

• Increasing/decreasing the "Available" quantity will also automatically increase/decrease the "On Hand" quantity in Shopify by the same amount, which is why you don't see separate options for Increase/Decrease On Hand Qty (the Shopify API actually doesn't allow this).  So if you need to Increase/Decrease On Hand Qty, simply use the Increase/Decrease Available Qty options. 
• Shopify uses the following calculations to automatically adjust quantity values (official docs):
  • Available = On Hand - Committed
  • On Hand = Available + Committed + Unavailable