How can customer pre order products that are out of stock?

Pre Ordering products can work great with Sumtracker.

Shopify gives you the flexibility to keep selling products that are out of stock. 

When you turn this setting on, customer can keep buying the products when they are out of stock.

As you get orders for this product, you can know exactly how much quantity has been pre-ordered by checking the Booked quantity in Sumtracker.

Sumtracker will keep syncing the inventory to Shopify. 

When the stock of the product arrives, you can add it to Sumtracker to show the correct In Stock and the inventory Available to sell.

When you want to turn off pre-ordering, you can go to the Shopify listing and uncheck the 'Continue selling when out of stock' option.