Import Costs

My customer wants me to include import costs in the item average cost. We do not know the costs at time of receipt nor do we necessarily know what it is at payment. It might be a month before we receive the documentation, Is there a way we can add this cost to inventory after the fact, with the realization that the inventory may be gone before we get the paperwork? I know this sounds like landed cost, but we don't own the module.

Thank you for your help.

Bob