This deal might have already been settled?
If not, I guess a transaction will be finalized first before funds and goods are actually sent. The full payable amount in BTC will be agreed before the buyer will send the full payment through a trusted escrow agreed by both parties. There may actually be no need for an advance.
I had proposed a price in BTC based on the best estimate of the cost of the product + cost of shipping + my own service fee, and I would have covered any unexpected variation. I had found an escrow and wrote an agreement in which the buyer would have covered the escrow fee, Bitcoin network transaction fees, and any custom tax in his country.
The buyer did not go forward with the deal after that point.
That deal sounds fair to me. But there must be a good reason for the other party to consider not proceeding.
Anyway, my response came a bit late.