@David4680 The currency conversion charge is usually more like 3%, not 30%. Sounds like something's wrong here and all I can suggest is that you be persistent with Airbnb in getting to the bottom of it.
Also, and you may already be totally aware of this, when you look at currency conversions for the day online, what you see is the mid-market rate, which is not the rate your currency would be converted at, as the entity that converts the funds, whether it is a bank, Airbnb, or whatever, will always add their cut- what you have to look at are the buy/sell rates for that day. This is just the rate at which the funds will be converted, and is separate from a fee for converting. If one of my work clients wires funds to my Mexican bank account in US dollars, that money is converted at the buy/sell rate for US dollars that the bank is using that day, and additionally they charge a $20US "conversion fee". If the client first converts the money online through a currency conversion site (with the client subject to the buy/sell rates, not me) and sends it through them, the money arrives in Mexican pesos and I avoid that $20 charge.
It probably would have been better financially to use one of those sites to wire or EFT $ directly to the host's bank account using one of those online services ( Transferwise and XE Trade are two examples) but also risky if the host wasn't honest and just kept the money without sending the jacket. Doing it through Airbnb means they have a record of the exchange.