Within the Shopify framework, products cannot have multiple price points. To work around this, Quantity Breaks creates variants for each price tier. After all, it's the same product, sold at a different price based on how many items the customer has in their cart.
Quantity Breaks is designed to work in the cart; it will swap out the regularly priced variant, and replace it with the tiered pricing variant. The variants created by Quantity Breaks on their own do not have inventory tracking. When looking at the variants from within Shopify, the inventory will appear as N/A.
No, changing the inventory value on the Quantity Break variants will skew the actual stock on hand. If you have 10 in stock, those would be represented by the parent (1+) variant. If you add a stock count of 10 to the 2+, and 5 to the 3+, you are telling Shopify you have 30 on hand; 10 on each.
Inventory is still tracked, but not the usual way. About 5-15 minutes after purchase, Quantity Breaks will update the base variant stock.
Using the same scenario, when a customer buys five items, it is the 3+ variant of the product that is being purchased. Shortly after, the main (1+) variant stock will be adjusted from 10 down to 5.
There is always the potential for a product to be oversold, but it should only happen once. When a product with Quantity Breaks applied to it is purchased, it is the Quantity Break variant being purchased. Inventory is not directly applied to these variants (N/A to Shopify means that it is always available). After the sale, the main variant stock is reduced. If that stock reaches 0 or less, all the Quantity Break variants stock values change from N/A to 0. This restricts any more products from being oversold.
To allow sales of the app created variants again, such as when you restock the regular variants, you'll need to select the "Manually Sync QB Inventory" button, found on the "Quantity Breaks" screen.
Because the app variants are always created with a quantity of "N/A", you will need to select this button if the regular version of the variant already has an inventory of 0 to prevent overselling. Because the app variants are recreated whenever a Quantity Breaks group is changed, it's good practice to select the sync button after a new group is created or when an existing one is edited.
When a Quantity Break variant is purchased, the app can "listen" to see if it's a Quantity Break variant and reduce the stock. When an order is canceled, we don't have the same capability. When canceling the order, the stock will be returned to the Quantity Break variant, which often will be N/A. The only workaround here is to manually add the inventory back to the main variant.