I am thinking to apply for Spirit Airlines credit card but heard they charge fees for miles redemptions, is that true?

Yes, there are situations when Spirit credit card holders are charged redemption fees on top of any taxes or fees associated with flight. When you use your miles for an award flight within less than 180 days, you will be charged a redemption fee. The amount of this fee varies depending on how close the flight date to the redemption date and ranges between $15 and $100. If you do not want to pay the redemption fees then you should either book your flight as far in advance as possible or choose another card. For example, a travel rewards credit card which is not tied to a particular airline won't charge you redemption fees.