Note: Do not delete the gift voucher option. Of a gift voucher is used and the total becomes zero your form will fall over and not function correctly.
This is auto selected and used in this case.
The new free option is e same. Even without gift voucher if you had a free item the gift voucher option used to be selected. Now it's the free one that is used in this case.
Like Florin said it depends on what your using and doing.
Europe, Middle East and Africa