Hi!
You are talking about a pattern "modern" for cart?
Code is embedded not in the cart
For example template "Default"
OPEN FILE - domainchecker.tpl
FIND
<div class="domcheckertldselect hide" id="tlds">
REPLACE
<div class="domcheckertldselect hide" id="tlds">
<div><input type="checkbox" name="total" value="checkbox" onClick="check_all_tld(this.form,'tlds[]',this.checked)" />All TLDs</div>
ADD in file /templates/default/js/whmcs.js string:
function check_all_tld(oForm, cbName, checked) { for (var i=0; i < oForm[cbName].length; i++) oForm[cbName].checked = checked; }