Why does Shopify still send its normal order confirmation for preorders? (Essent Preorder Back in Stock app)

The app doesn't replace or edit Shopify's own order confirmation. Shopify always sends it. The app sends a second email, the Preorder confirmation email, that tells the customer the order contains preorder items.

Turn on and edit the Preorder confirmation email

1. Go to Preorder → Settings → Notifications → Preorder confirmation email → Customize.
2. Click Turn on.
3. Edit the subject, logo, store name, description and payment labels. Use {orderName} for the order number customers see (for example #1042). Only {order} and {orderName} work.
4. Save. To test, place a test order for a preorder product.

This email needs the Lite plan or higher.

Wrong delivery estimate in Shopify's email or checkout?

Delivery times like "1–3 business days" come from your Shopify shipping settings, not from the app. To tell customers the preorder date, put it in the campaign's preorder message and preorder note (shown in cart, checkout and transactional emails), and in the Preorder confirmation email text.

Full guide: Preorder emails: confirmation and shipping update.