Delivery costs don't depend on the size of the order. However, service fees, while remaining between $2 and $4, vary depending on the size of the order.
That said, you have to understand that, even before placing an order and even before choosing the first dish, the delivery costs are very clearly indicated on the restaurant's page. The same is true for service fees, which can indeed vary from $2 to $4. Before confirming the order, the customer will see, for example, that the service fee is $2.59. Those fees are clearly displayed before payment.