Speaker Sales on Audiogon

I recently listed my Tyler Acoustics for sale with the “buy now option” but had checked the box stating that shipping costs would be assessed after the sale, since I didn’t know how much to charge for shipping because I didn’t know where they’d ship to. I also stated this is my ad.  
Well, a gentleman bought them and he paid me an extra $100 without talking to me about shipping costs, but shipping these to his location is $300 via fed ex...  I sold the speakers at a low price of $699 and can’t justify $300 in shipping.  What do I do?

Also, Is it normally around $50 to sell something on Audiogon?  I paid all of my listing fees upfront ($30) and then Audiogon took another $20 when the item sold as a transaction fee, and on top of that, it looks like I still owe Audiogon $20...