Changing Radio Buttons into Checkbox - Shipping

Hello!

I want to provide multiple options for shipping - say : Free Shipping (since items are inclusive of shipping charges) + a checkbox so people can choose the option to opt for recorded delivery / registered shipping. I was thinking of configuring the Flat Rate extension by changing title, since I need to define only one price for the registration.

Can somebody tell me which files do I need to edit?

Thanks

Ok, I found a way, but when I choose 2x shipping options, the grand TOTAL still shows 1x shipping option.

Again, the query is that I want to provide an optional feature where people can opt for first class delivery.

Thanks

this is the exact same thing im trying to solve

It doesn't really make sense that way. You can only choose one shipping option. If you want added fees or options you'd need something like this:
http://www.opencart.com/index.php?route ... on_id=3107

That will allow you to have an additional checkbox during checkout to add optional fees