How Do Vouchers and Credits Work?
 

I have a credit from a cancelled flight. Am I allowed to use this on add-ons like seat fees?

I also have a voucher for $100 that Frontier gave me as a customer service gesture. I know that it only applies to base fare. Can this be combined with the flight credit?

Typically only on base fare and only for you. No add-ons nor taxes nor CIC or other fees. Typically you can combine them.

So my flight credit can only be used on base fare despite the credit also containing value for the bundle that I purchased?

I've booked some flights to try to use up my $100 voucher that expired in a few weeks. Luckily, one flight that I wanted used up $25 of the credit. One flight with two segments only used up 74 cents bc that was the airfare portion.

I have one or two more trips that I might book and there's a decent chance I won't use up the remaining $50 voucher credit I have.

I don't think anyone fully understands how Frontier vouchers and credits work, but if you have a service interruption, I would use the chat or phone call feature to make sure you get a voucher that is good for all ALL costs of a new ticket -- not just the (usually modest) base fare. They can do this. I know because they've now done it for me a couple of times, after previously giving me a voucher that was only saving me like a $1.40 on my next ticket (with a short voucher expiration date no less!).

You also can't use the Voucher on the app, only on the website.

Earlier this year had a FLL flight running very late so I cancelled and took another airline. I do I use the money on that confirmation code which was never used. Wanted to book a flight now saw no field to use the trip I cancelled.