Going over your Monthly Active Contact (MAC) limit on a Pro or Premium plan used to mean an unplanned upgrade to a higher-cost tier, even if the overage was temporary (for example, a one-off campaign that brings in a spike of contacts for a single cycle).
Now, card-paying users on Pro or Premium plans get incremental 100 MAC add-ons purchased automatically the moment they cross their limit, rounded up to the nearest 100. The charge applies only to the current billing cycle and resets on the next billing date, so your permanent MAC limit stays the same.
If you'd rather avoid this per-cycle charge, you can proactively buy a permanent MAC add-on to raise your base limit instead.
Worth knowing: auto-scaling can't be turned off for card-paying users. If you pay by invoice or don't have a card on file, nothing changes for you; you'll still see manual add-ons and a hard block at the limit as before.