Uh oh – you have sold to Brazil – that’s a no-no. Selling to any South American country can be problematical but especially Brazil, where the customs duty is collected on arrival (and may be “increased” it seems at the discretion of the Customs agents.) If the buyer was a regular they should have known that and have been prepared for the extra cost.

OR maybe he was relying on the fact that you were not a big seller and hoping to obtain a refund from you.

See this thread

https://community.ebay.com.au/t5/Selling/customs-and-import-duties/td-p/2575068

for information that you should be able to present to eBay in the dispute resolution process along with tracking showing the item has arrived at Customs; although the above thread is 12 months old I would hope the information is still current. I would hang tough; present all this information to the dispute immediately; do NOT offer any refund; you should be able to contact eBay on Monday.  

I do hope this works out for you. If you decide to continue to sell Internationally do restrict your potential buyers to places that seem the least problematical. There are at present 142 locations I don’t sell to.  As I said in the thread quoted above "If you sell to places where eBay collects the taxes up front (like GB, USA (most states), EU, NZ - there should be a list somewhere but I don't know where), then you do not have to face this problem." 

You obviously have the tracking number - upload it to the dispute.

The buyer agreed to the sale when paying.