I beleive those codes are one-time use 2500 miles coupon codes given out during flight if you sign up for the Spirit Airlines credit card. Codes are redeemable at www.spirit.com/2500 limit of one per account. The variables in the string seem to match up ( I checked them out on Retailmenot)

I do not think there are any current bonus miles incentives going on for Spirit bookings at the moment, just the usual discounts in the emails they send out regularly.