// Add trailing slashes ONLY to buttons
add_action('wp_footer', 'add_slash_to_buttons_only');
function add_slash_to_buttons_only() {
    echo '<script>
    document.addEventListener("DOMContentLoaded", function() {
        var buttons = document.querySelectorAll("a.cheapest-action, a.btn, a.button, button a");
        for (var i = 0; i < buttons.length; i++) {
            var btn = buttons[i];
            var href = btn.getAttribute("href");
            
            if (!href || href.charAt(0) !== "/") continue;
            if (href.charAt(href.length - 1) === "/") continue;
            if (href.match(/\.(jpg|jpeg|png|gif|pdf|zip|css|js|svg|ico)$/i)) continue;
            if (href.indexOf("?") > -1 || href.indexOf("#") > -1) continue;
            
            btn.setAttribute("href", href + "/");
        }
    });
    </script>';
}