includes/class-tmdo-wc-orders-interceptor.php

@@ -149,6 +149,10 @@ class TMDO_WC_Orders_Interceptor extends TMDO_Interceptor_Base {
 		if ( ! get_option( 'hp_vendor_include_taxes' ) ) {
 			$subtotal -= (float) $order->get_total_tax();
 		}
+		// Marketplace subtracts the full tax even after a refund; a fully
+		// refunded taxed order would otherwise record a negative amount.
+		$subtotal = max( 0.0, $subtotal );
+
 		$vendor_payout   = max( 0.0, $subtotal * $vendor_share - $fee );
 		$commission      = $subtotal - $vendor_payout;
 		$commission_rate = round( ( 1 - $vendor_share ) * 100, 2 ); // Platform percentage, as set in Marketplace.

tests/integration/WCOrdersInterceptorTest.php

@@ -204,6 +204,23 @@ class WCOrdersInterceptorTest extends TestCase {
 		$this->assertEqualsWithDelta( 0.0, (float) $row['vendor_payout'], 0.0001 );
 	}
 
+	/**
+	 * Amounts follow Marketplace::get_order_profit() (what the vendor is paid,
+	 * which subtracts the full tax after refunds), but a fully refunded taxed
+	 * order must not record a negative subtotal or commission.
+	 */
+	public function test_a_fully_refunded_taxed_order_records_no_negative_amounts(): void {
+		$this->place_order( $this->marketplace_meta(), 110.0, 10.0 );
+		$this->refund( 110.0, 'refunded' );
+
+		( new TMDO_WC_Orders_Interceptor() )->action_order_refunded( self::ORDER_ID, 1 );
+
+		$row = $this->commission_row();
+		$this->assertEqualsWithDelta( 0.0, (float) $row['subtotal'], 0.0001 );
+		$this->assertEqualsWithDelta( 0.0, (float) $row['commission'], 0.0001 );
+		$this->assertEqualsWithDelta( 0.0, (float) $row['vendor_payout'], 0.0001 );
+	}
+
 	public function test_a_refund_of_an_untracked_order_records_nothing(): void {
 		$this->refund( 50.0, 'pending' );