Nothing will happen, BUT:

1. If the customer decides to unsubscribe, you get one hour to charge them from the moment you get the SNS notification. That mean that if there is a failure in the submission, you won't get paid...
2. The customer will not have visibility on the spend until you submit the metering. They might dispute the amount, or decide that it's too costly. This might result in bad customer experience.
3. As you scale to more and more customers, you would need to send more API requests (There is a limit to the usage records sent in the API), and you might want to create more dimensions. Consider it in your design.