Ensuring that add-on item ships with pre-order even if cart contains other in-stock items?

If you are selling pre-orders via the Purple Dot checkout, and using the version of Purple Dot checkout that supports selling regular in-stock items alongside pre-order line items (aka "mixed" or "combined" carts), then you can force the PD checkout to attach some in-stock product to the first pre-order shipping group by giving it the pd-add-on-item tag in Shopify:

Adding this tag prevents the item from being treated as an individual in-stock product and exported into Shopify right when the order is created. 

Note: This functionality only applies to brands using our combined checkout, which allows in-stock and pre-order items to be purchased together.