単純にプラグインで実装可能です。

https://wordpress.org/plugins/last-viewed-posts/

プラグインをダウンロードしてきてlast_viewed_posts.phpを編集しましょう。主な設定項目は少ないのでザックリ解説していきます。

$zg_cookie_expire = 30; // After how many days should the cookie expire? Default is 360.
$zg_number_of_posts = 4; // How many posts should be displayed in the list? Default is 10.
$zg_recognize_pages = false; // Should pages to be recognized and listed? Default is true.

41行目あたりを上記のように設定。クッキーの保有期間を30日、一覧表示は4件、商品カテゴリのみを表示。という感じの設定になります。

function zg_lwp_header() { // Main function is called every time a page/post is being generated
    if (is_single()) {
        zg_lw_setcookie();
    } else if (is_page()) {
        global $zg_recognize_pages;
        if ($zg_recognize_pages === true) {
            zg_lw_setcookie();
        }
    }
}

次に上記のコードを見つけます。47行目くらいにあるはずです。
これをWelcart用に書き換えます。

function zg_lwp_header() { // Main function is called every time a page/post is being generated
	if (is_single() && in_category('item')) {
		zg_lw_setcookie();
	} else if (is_page()) {
		global $zg_recognize_pages;
		if ($zg_recognize_pages === true) {
			zg_lw_setcookie();
		}
	}
}

特に

if (is_single() && in_category(‘item’))

この箇所、最後のカッコが抜けるとエラーで動きませんので注意しましょう。最後に90行目くらいの出力場所を編集。

function zg_recently_viewed() { // Output
    echo '<ul class="viewed_posts">';
    if (isset($_COOKIE["WP-LastViewedPosts"])) {
        //echo "Cookie was set.<br/>";  // For bugfixing - uncomment to see if cookie was set
        //echo $_COOKIE["WP-LastViewedPosts"]; // For bugfixing (cookie content)
        $zg_post_IDs = unserialize(preg_replace('!s:(\d+):"(.*?)";!e', "'s:'.strlen('$2').':\"$2\";'", stripslashes($_COOKIE["WP-LastViewedPosts"]))); // Read serialized array from cooke and unserialize it
        foreach ($zg_post_IDs as $value) { // Do output as long there are posts
            global $wpdb;
            $post = get_post($value);//Welcart用
            $zg_get_title = $wpdb->get_results("SELECT post_title FROM $wpdb->posts WHERE ID = '$value+0' LIMIT 1");
            foreach($zg_get_title as $zg_title_out) {
                echo "<li><a href=". get_permalink($value+0) . " title=". $zg_title_out->post_title . ">". usces_the_itemImage($number=0, $width=200, $height=200, $post, 'return'). "<span>". $zg_title_out->post_title. "</span></a></li>\n"; // Welcart 商品を出力
            }
        }
    } else {
        //echo "No cookie found.";  // For bugfixing - uncomment to see if cookie was not set
    }
    echo '</ul>';
}

商品個別ページのテンプレートに以下を書き加えて完了です。

<?php if (function_exists('zg_recently_viewed')):  if (isset($_COOKIE["WP-LastViewedPosts"])) { ?>
 <h2>最近見た商品</h2>
 <?php zg_recently_viewed(); ?>
<?php }  endif; ?>