Folks running subscription based business with hard limits on specific actions, a question for you.

If you are currently running a subscription based business that limits certain actions based on the payment plan, how are you handling when a user nears or exceeds that limit?

For example, let's say your business is a SaaS for sending emails. You offer several payment tiers, the lowest tier limits the user to 1k emails. A user has just hit 1k emails, how should you respond to email request 1001?

I've considered the following options:

1. Don't allow the user to send any more emails past the limit. Notify them of the limit being hit and that they should upgrade.
2. Allow them to exceed the limit but charge a fee per email sent over the limit. That fee will be higher that the average cost of the 1k already sent that was within the plan's limits.
3. Allow them to exceed the limit and not charge extra. Notify them of the limit being hit and that they should upgrade.

Scenario 1 doesn't sit right with me as it's leaving business on the table. I like the scenario 2, but would need to dive deeper into Stripe to see how subscriptions could handle the extra fees. Scenario 3 seems to be the best for garnering good faith amongst users but could lead to abuse.

Would love to hear more thoughts.