Accruing 'cost of goods' for outbound freight

We currently receive bills for our outbound freight to customers weeks after the sales have occurred. We're trying to find a way to estimate the cost of freight to give our sales team better visibility of margins when making sales orders.

The thought I had was to add a service/other line item to each sales order, which had a fixed cost price, and a zero sales price (since we don't charge our customers for freight).

I had hoped this would allow me to accrue a cost of goods or expense account automatically, which I could later draw down on when I received the bill, but it hasn't worked out for me, as the service/other line items doesn't generate any posting transactions.