Force Free Checkout when Zero Total

This VQMOD allows 'Free Checkout' to appear ONLY when the Shopping Cart Total = $0.

This is useful if you only want Free Checkout when total = 0.

If you allow free checkout on opencart, it will ALWAYS appear. :(

This VQMOD makes it ONLY appear when total is 0. When total is not 0, free checkout does NOT appear.

Good if you have a lot of 'Store Credit' that pays for items to make total 0.

All credits go to imdevlper18.

Tested on 1.5.6.X