Can you insert the quoted shipping amount in the incomplete invoice? Do NOT hit Recalculate.
If you can, then pay it.
Hmm- can you see any invoice? Pay it. If there is no shipping or if the shipping is too low, the seller will likely jump to correct it.
If the seller wants to complete the sale, he will have to send you a correct invoice. If not, you can start an Item Not Recieved Dispute.