Here's some data for Dec 2024: I booked Spirit flights for a group trip and later added seat selection.

• 12/7: Bought Saver's Club membership ($69.95), individual ticket ($40.18), individual ticket ($38.20), and two tickets ($80.36)
• 12/8: Bought 2 seat selections separately ($20.00, $18.00)
• 12/10: Billing period closed
• 12/11: Bought two seat selections in one purchase ($40)
• 12/12: Received credits for membership (-$69.95), and the individual tickets (-$40.18 and -$38.20)
• 12/13: Tracker has updated to show $186 / $200 earned, so I expect the seat selections from 12/8 will appear soon, and hopefully the two seats from 12/11 will post soon to fully use up the credit. Will edit