Line 232 in the SagePayPaymentProcessor class.
if (order.OrderShippingInclTax > 0.0M)
{
// use code 999999 as the delivery code
toReturn.AppendFormat(":Delivery:999999:{0:0.00}:{1:0.00}:{2:0.00}:{3:0.00}",
order.OrderShippingExclTax,
order.OrderShippingInclTax - order.OrderShippingExclTax,
order.OrderShippingInclTax,
order.OrderShippingInclTax
);
}