For years I just uploaded a lump sum and it was spent as I used the service per year. This way I didn't need to bother what I would be without email when the paid period is over and I didn't need to overpay much in case I would need to cancel early.
Now I need to be sure I be around with a working CC at the time the bought period is over... and what if won't? Do I need to jump through cancel-and-reorder hoops every couple of years? More importantly, am I sure what a couple years later these hoops would work or through some very unlucky coincidence it would wipe my decade+ emails? Sure, Fastmail guys would be so sorry but... that wouldn't help me.