wc_vendors_custom_commission_country.php

<?php

add_filter('wcv_commission_rate', 'm_commission_update', 20, 5);

/**
 * @param $commission
 * @param $product_id
 * @param $product_price
 * @param $order WC_Order
 * @param $qty
 *
 * @return float
 */
function m_commission_update($commission, $product_id, $product_price, $order, $qty) {

    $countries = array('US', 'UK');

	$author = WCV_Vendors::get_vendor_from_product( $product_id );

	if ( WCV_Vendors::is_vendor( $author ) ) {

		// check the order location
		if ( in_array($order->get_billing_country(), $countries) ) {

		    // the commission you want to add - switch to whatever logic fits
			$card_commission_rate = 5;
			// add the contry specific commission
			$card_commission = $product_price * ( intval( $card_commission_rate ) / 100 );
			$card_commission = round( $card_commission, 2 );

			// notice the + - add to the actual commission so they get more money
			$commission += $card_commission;

		}

	}

	return $commission;

}