This feature is available to merchants on our Growth Plan with Shopify Plus.
In your Shopify admin, go to Settings and click Checkout.
In the Configurations section, click Customize next to the checkout you want to edit.
At the top menu bar, select the checkout page where you want to add the app.
Click Add app block.
In the Apps available for this page section, click Magical Product Fees. The app block will appear in the recommended area of your checkout.
Click the Magical Product Fees block.
Under Checkout behavior, turn on Include app block in Shop Pay.
Also under Checkout behavior, turn on Allow app to block checkout.
Click Save.