Rakuten has the grey market Canon 1.4x III extender for sale from 6th Avenue Electronics for $378.49. When you purchase using ApplePay via Rakuten.com on Safari on an apple device you get an additional $15 off. You then get 20% back ($73) in Rakuten Super Points.
$378.49 - $15 = $363.49
$363.49 * 20% = $72.70.
Seems like a good deal compared to the sale prices of the same item on Ebay which are in the $370 - $385 range.