WooCommerce Pre-Orders Plugin

I'm looking into purchasing WooCommerce Pre Orders and have been looking at a friend's copy to demo. It's a great plugin, but I have an issue I can't seem to solve.

When ordering, you can only order 1 variation of a product at a time. You can edit the quantity of that variation in the cart but you can't add 2 variations of the same product to the same cart at the same time. If you do, it will empty the cart and replace it with the current selection. Since I'd be taking pre-orders for screen printed clothing (similar to teespring), ordering multiple variations (sizes in this instance) at one time is important. Making them make multiple orders from the same product would just drive them away.

I don't want to let customers order from multiple preorders at once since each preordered product has a different release/ship date, but I want to let them order multiple variations, i.e. a Small Tee, a Medium Tee, and a Large Tee, of a particular product since they would all ship at the same time.

I hope all of that made sense.

Here is the code that is responsible for the cart restrictions. Any help is much appreciated.

/** * When a pre-order is added to the cart, remove any other products * * @since 1.0 * @param bool $valid * @param $product_id * @return bool */ public function validate_cart( $valid, $product_id ) { global $woocommerce;

    if ( WC_Pre_Orders_Product::product_can_be_pre_ordered( $product_id ) ) {

        // if a pre-order product is being added to cart, check if the cart already contains other products and empty it if it does
        if( $woocommerce->cart->get_cart_contents_count() >= 1 ) {

            $woocommerce->cart->empty_cart();

            $string = __( 'Your previous cart was emptied because pre-orders must be purchased separately.', 'wc-pre-orders' );

            // Backwards compatible (pre 2.1) for outputting notice
            if ( function_exists( 'wc_add_notice' ) ) {
                wc_add_notice( $string );
            } else {
                $woocommerce->add_message( $string );
            }
        }

        // return what was passed in, allowing the pre-order to be added
        return $valid;

    } else {

        // if there's a pre-order in the cart already, prevent anything else from being added
        if ( $this->cart_contains_pre_order() ) {

            // Backwards compatible (pre 2.1) for outputting notice
            if ( function_exists( 'wc_add_notice' ) ) {
                wc_add_notice( __( 'This product cannot be added to your cart because it already contains a pre-order, which must be purchased separately.', 'wc-pre-orders' ) );
            } else {
                $woocommerce->add_error( __( 'This product cannot be added to your cart because it already contains a pre-order, which must be purchased separately.', 'wc-pre-orders' ) );
            }

            $valid = false;
        }
    }

    return $valid;
}


/**
 * Add any applicable pre-order fees when calculating totals
 *
 * @since 1.0
 */
public function maybe_add_pre_order_fee() {
    global $woocommerce;

    // Only add pre-order fees if the cart contains a pre-order
    if ( ! $this->cart_contains_pre_order() ) {
        return;
    }

    // Make sure the pre-order fee hasn't already been added
    if ( $this->cart_contains_pre_order_fee() ) {
        return;
    }

    $product = self::get_pre_order_product();

    // Get pre-order amount
    $amount = WC_Pre_Orders_Product::get_pre_order_fee( $product );

    if ( 0 >= $amount ) {
        return;
    }

    $fee = apply_filters( 'wc_pre_orders_fee', array(
        'label' => __( 'Pre-Order Fee', 'wc-pre-orders' ),
        'amount' => $amount,
        'tax_status' => WC_Pre_Orders_Product::get_pre_order_fee_tax_status( $product ), // pre order fee inherits tax status of product
    ) );

    // Add fee
    $woocommerce->cart->add_fee( $fee['label'], $fee['amount'], $fee['tax_status'] );
}


/**
 * Checks if the current cart contains a product with pre-orders enabled
 *
 * @since 1.0
 * @return bool true if the cart contains a pre-order, false otherwise
 */
public static function cart_contains_pre_order() {
    global $woocommerce;

    $contains_pre_order = false;

    if ( ! empty( $woocommerce->cart->cart_contents ) ) {

        foreach ( $woocommerce->cart->cart_contents as $cart_item ) {

            if ( WC_Pre_Orders_Product::product_can_be_pre_ordered( $cart_item['product_id'] ) ) {

                $contains_pre_order = true;
                break;
            }
        }
    }

    return $contains_pre_order;
}


/**
 * Checks if the current cart contains a pre-order fee
 *
 * @since 1.0
 * @return bool true if the cart contains a pre-order fee, false otherwise
 */
public static function cart_contains_pre_order_fee() {
    global $woocommerce;

    foreach ( $woocommerce->cart->get_fees() as $fee ) {

        if ( is_object( $fee ) && 'pre-order-fee' == $fee->id )
            return true;
    }

    return false;
}


/**
 * Since a cart may only contain a single pre-ordered product, this returns the pre-ordered product object or
 * null if the cart does not contain a pre-order
 *
 * @since 1.0
 * @return object|null the pre-ordered product object, or null if the cart does not contain a pre-order
 */
public static function get_pre_order_product() {
    global $woocommerce;

    if ( self::cart_contains_pre_order() ) {

        foreach ( $woocommerce->cart->cart_contents as $cart_item ) {

            if ( WC_Pre_Orders_Product::product_can_be_pre_ordered( $cart_item['product_id'] ) ) {

                // return the product object
                return get_product( $cart_item['variation_id'] ? $cart_item['variation_id'] : $cart_item['product_id'] );
            }
        }

    } else {

        // cart doesn't contain pre-order
        return null;
    }
}