Which payment methods work with preorders (PayPal, Klarna, Shop Pay)? (Essent Preorder Back in Stock app)

• By default, preorders use a Shopify selling plan (purchase option). Shopify only supports these with Shopify Payments and PayPal Express. Local methods such as Klarna, iDEAL, Sofort or Mollie aren't supported for preorders. This is a Shopify rule.
• Partial payments (deposits) need Shopify Payments.
• Express checkout buttons (Shop Pay, Apple Pay, Google Pay) and Shop Pay Installments aren't supported by Shopify for products sold with a purchase option, so they're hidden on preorder products.
• Store credit can't be used on deferred purchases. This is also a Shopify rule.

Need other gateways?

Turn off the selling plan for that campaign: campaign → Advanced settings (bottom of the Content tab) → untick Use Shopify selling plan (purchase option) → Save. Preorders then check out as normal one-time purchases with your other payment methods, but partial payments, app discounts and On hold / Scheduled fulfilment are switched off for that campaign.

Full guide: Turn off selling plans for a preorder campaign.