A search should have found the answer, but...

Online, you can only use it for an airfare >=$350. That doesn't include taxes, which you'll need to pay on top of the fare.

You can use it on a fare less than the voucher amount, but you'll need to do it over the phone, and you'll lose any extra value - plus you'll still need to pay taxes. ie, book a $200 fare with $20 in taxes and it'll cost you your entire $350 voucher, plus $20.