shipping amount changed on invoice

[downsizing2018]

I recently sold an item - at a quick glance the buyer was from the USA.  The shipping should have been $28.00  Then I get a message from the buyer requesting a quote for shipping to a Canadian address.  I also get a receipt for a payment of the auction item and the invoice states "free shipping", and the address is now a Canadian address.  I DID NOT offer free shipping to Canada, I asked for $18.00.

First off, any idea how this happened?

and

Do I have any recourse if the buyer does not pay the request for 18.00 that I sent through paypal?

[downsizing2018]

Okay, all is well, the buyer paid the appropriate shipping, but the question for me still remains, how did this happen? How could the invoice just arbitrarily get switched to free shipping?