To send a gift card if the order value is above $500, you can use the template "Add BOGOS gift to eligible orders" (https://admin.shopify.com/apps/flow/editor/templates/0190cdfa-617c-735e-92d9-f3c085d6789a) as a reference. However, this template is specifically designed for the BOGOS app, so you would need to modify it to fit your gift card requirements.
Here's a general outline of the steps you could take:
You can also use the "orderEditAddCustomItem" mutation from the send-admin-api-mutations.json document to add a custom line item (in this case, a gift card) to an existing order.
Please note that the exact implementation may vary depending on your specific requirements and the Shopify Flow setup.