Сделать продукт для покупки только один раз или нет, если какой-либо элемент из определенной категории продукта был куплен в Woocommerce.

function has_bought_items( $user_id = 0, $product_ids = 0, $categories_slugs = '' ) {
    global $wpdb;
    $customer_id = $user_id == 0 || $user_id == '' ? get_current_user_id() : $user_id;
    $statuses    = array_map( 'esc_sql', wc_get_is_paid_statuses() );

    if ( is_array( $product_ids ) ) {
        $product_ids = implode(',', $product_ids);
    }

    if ( $product_ids !=  ( 0 || '' ) ) {
        $query_in = "IN ($product_ids)";
    }
    else {
        $categories = is_array( $categories_slugs ) ? implode("','", $categories_slugs) : $categories_slugs;

        $query_in = "IN (
        SELECT DISTINCT products.ID  FROM {$wpdb->prefix}posts AS products
        INNER JOIN {$wpdb->prefix}term_relationships as term_rel
            ON products.ID = term_rel.object_id
        INNER JOIN {$wpdb->prefix}term_taxonomy as term_tax
            ON term_rel.term_taxonomy_id = term_tax.term_taxonomy_id
        INNER JOIN {$wpdb->prefix}terms as terms
            ON term_tax.term_taxonomy_id = terms.term_id
        WHERE products.post_status = 'publish'
        AND term_tax.taxonomy = 'product_cat'
        AND terms.slug IN ('$categories')
        )";
    }

    // Count the number of products
    $product_count_query = $wpdb->get_var( "
        SELECT DISTINCT COUNT(orders.ID)
        FROM {$wpdb->prefix}posts AS orders
        INNER JOIN {$wpdb->prefix}postmeta AS order_meta
            ON orders.ID = order_meta.post_id
        INNER JOIN {$wpdb->prefix}woocommerce_order_items AS order_items
            ON orders.ID = order_items.order_id
        INNER JOIN {$wpdb->prefix}woocommerce_order_itemmeta AS order_itemmeta
            ON order_items.order_item_id = order_itemmeta.order_item_id
        WHERE orders.post_status IN ( 'wc-" . implode( "','wc-", $statuses ) . "' )
        AND order_meta.meta_key = '_customer_user'
        AND order_meta.meta_value = $customer_id
        AND order_itemmeta.meta_key IN ( '_product_id', '_variation_id' )
        AND order_itemmeta.meta_value $query_in
    " );

    // Return a boolean value if count is higher than 0
    return $product_count_query > 0 ? true : false;
}

Ваш обновленный код с использованием пользовательской условной функции выше:

// add_filter( 'woocommerce_variation_is_purchasable', 'trial_package_purchasable_once', 10, 2 );
add_filter( 'woocommerce_is_purchasable', 'trial_package_purchasable_once', 10, 2 );
function trial_package_purchasable_once( $purchasable, $product ) {
    // HERE set the free trial product Id to be purchased only once (can be an array of Ids too)
    $free_trial_id = 50;

    // HERE set the specific product category SLUG (could be an array of slugs too)
    $categories = array('lettering'); // Only slug terms

        if ( $product->get_id() == $free_trial_id ) {
            // Check if any item of the specific product category has already been purchased once
            if ( has_bought_items( get_current_user_id(), '', $categories ) ) {
                return false;
            }
            // Check if the free trial product has already been purchased once
            elseif ( has_bought_items( get_current_user_id(), $free_trial_id ) ) {
                return false;
            }
        }
    }

    return $purchasable;
}

Код находится в файле function.php вашей активной дочерней темы (или активной темы). Проверено и работает.

Для обработки категорий продуктов Term ID вместо Term Slugs используйте вместо этого следующую условную функцию:

// For Product category(ies) Id(s)
function has_bought_items( $user_id = 0, $product_ids = 0, $categories_ids = '' ) {
    global $wpdb;
    $customer_id = $user_id == 0 || $user_id == '' ? get_current_user_id() : $user_id;
    $statuses    = array_map( 'esc_sql', wc_get_is_paid_statuses() );

    if ( is_array( $product_ids ) ) {
        $product_ids = implode(',', $product_ids);
    }

    if ( $product_ids !=  ( 0 || '' ) ) {
        $query_in = "IN ($product_ids)";
    }
    else {
        $categories = is_array( $categories_ids ) ? implode(',', $categories_ids) : $categories_ids;

        $query_in = "IN (
        SELECT DISTINCT products.ID  FROM {$wpdb->prefix}posts AS products
        INNER JOIN {$wpdb->prefix}term_relationships as term_rel
            ON products.ID = term_rel.object_id
        INNER JOIN {$wpdb->prefix}term_taxonomy as term_tax
            ON term_rel.term_taxonomy_id = term_tax.term_taxonomy_id
        INNER JOIN {$wpdb->prefix}terms as terms
            ON term_tax.term_taxonomy_id = terms.term_id
        WHERE products.post_status = 'publish'
        AND term_tax.taxonomy = 'product_cat'
        AND terms.term_id IN ($categories)
        )";
    }

    // Count the number of products
    $product_count_query = $wpdb->get_var( "
        SELECT DISTINCT COUNT(orders.ID)
        FROM {$wpdb->prefix}posts AS orders
        INNER JOIN {$wpdb->prefix}postmeta AS order_meta
            ON orders.ID = order_meta.post_id
        INNER JOIN {$wpdb->prefix}woocommerce_order_items AS order_items
            ON orders.ID = order_items.order_id
        INNER JOIN {$wpdb->prefix}woocommerce_order_itemmeta AS order_itemmeta
            ON order_items.order_item_id = order_itemmeta.order_item_id
        WHERE orders.post_status IN ( 'wc-" . implode( "','wc-", $statuses ) . "' )
        AND order_meta.meta_key = '_customer_user'
        AND order_meta.meta_value = $customer_id
        AND order_itemmeta.meta_key IN ( '_product_id', '_variation_id' )
        AND order_itemmeta.meta_value $query_in
    " );

    // Return a boolean value if count is higher than 0
    return $product_count_query > 0 ? true : false;
}

Это лучшая альтернатива для иностранных языков со специальными символами.