Improvements to Javascript to ensure cross-browser compatability (Prestashop and osCommerce sites) by NeilColeclough

I have 3 x Prestashop 1.4 sites and 1 x osCommerce 2.2 site. I need to be certain that the Javascript deployed on each of these sites (principally in the checkout process) is compatible with all versions of IE, Firefox, Chrome, Opera, and Safari… (Budget: £20-£250 GBP, Jobs: Javascript)