Can customers buy preorder and in-stock items in the same order? (Essent Preorder Back in Stock app)

Yes, by default. It stays one Shopify order with one shipping charge. What happens after checkout depends on the campaign's fulfilment status (campaign → Content → Shipping & mixed carts):

• Unfulfilled: one fulfilment. Everything can be shipped straight away.
• Scheduled or On hold: two fulfilments in the same order. Ship the in-stock items now, and the preorder items stay held until you release them or the date arrives.

Stop customers mixing them

Tick Don't allow mixed carts and write your own error message. Customers then have to place separate orders. This needs the Starter plan or higher.

Good to know

• The app never splits one order into two orders.
• It can't split one line into part in-stock and part preorder. If a customer orders 4 and you have 2, all 4 follow the same rule.
• If a mixed order still got through after you blocked mixed carts, contact support with the order number so we can check it.

Full guide: Mixed carts: preorder and in-stock items in one order.