protected function set_coupon( $order, $coupon, $action ) {

		// coupon amount must be positive float
		if ( isset( $coupon['amount'] ) && floatval( $coupon['amount'] ) < 0 ) {
			throw new WC_API_Exception( 'woocommerce_invalid_coupon_total', __( 'Coupon discount total must be a positive amount.', 'woocommerce' ), 400 );
		}

		if ( 'create' === $action ) {

			// coupon code is required
			if ( empty( $coupon['code'] ) ) {
				throw new WC_API_Exception( 'woocommerce_invalid_coupon_coupon', __( 'Coupon code is required.', 'woocommerce' ), 400 );
			}

			$item = new WC_Order_Item_Coupon();
			$item->set_props( array(
				'code'         => $coupon['code'],
				'discount'     => isset( $coupon['amount'] ) ? floatval( $coupon['amount'] ) : 0,
				'discount_tax' => 0,
				'order_id'     => $order->get_id(),
			) );
			$order->add_item( $item );
		} else {

			$item = new WC_Order_Item_Coupon( $coupon['id'] );

			if ( isset( $coupon['code'] ) ) {
				$item->set_code( $coupon['code'] );
			}

			if ( isset( $coupon['amount'] ) ) {
				$item->set_discount( floatval( $coupon['amount'] ) );
			}

			$coupon_id = $item->save();

			if ( ! $coupon_id ) {
				throw new WC_API_Exception( 'woocommerce_cannot_update_order_coupon', __( 'Cannot update coupon, try again.', 'woocommerce' ), 500 );
			}
		}
	}