How do you redem gift cards/groupon
 

I am at the payment page and I don't see anything to enter this information. If i call and book over the phone would i have to pay anything?

Okay, apparently the gift card option doesn't come up if you add an infant

There's no charge to book over the phone, unlike many airlines, so that's an option.

I have not used a Groupon, but have used gift cards online more than once:

The groupon code goes, I believe, into the Promo Code box. Click on "advanced search" to get a booking screen which includes the coupon code box.

The gift card you use at checkout -- you choose it as payment type, and then key in gift card number and PIN. If the card does not have enough value, you are then prompted for another payment method to cover the balance.

If the system is fighting you trying to use the gift card in a booking with an infant, it looks like you can book online w/o the infant, but then call to add the infant information.

Infant Guidelines

An infant is allowed to travel free of charge on the lap of an adult from seven days old until his or her second birthday. Even if your infant is flying for free, there may be taxes due when traveling internationally. International taxes will be collected at check-in.
Book all travelers EXCEPT the lap infant online then contact Reservations at 800-432-1359 to add the lap infant to the reservation.

I suppose then you may as well book the entire reservation w/Groupon and infant on the phone, since there is no longer a fee for phone reservations.

I recently attempted to use a Frontier promo card online. The system kept rejecting it. I called, the agent told me it could only be used online.

Nonetheless, she graciously gave me complimentary STRETCH seats for the inconvenience.