Order line items required for my Payment gateway to process payment.

[mac_dev]

Hi!

I am currently developing a Payment App extension. The payment gateway needs the order line items before it redirects to the Payment Gateway Page where the customer should pay the order.  Shopify initiates an HTTP POST request to payment gateway that contains information about the customer and the order see sample request in this link https://shopify.dev/docs/apps/payments/implementation/process-a-payment/offsite

Obviously there is no line items included in the request and that is what the payment gateway requires before it can redirect to the payment gateway page. Is there any way how to get the line items of the order?