Do data transfers via VPC peering count towards the data transfer allowance?

Yes, data transfers via VPC peering generally do count towards the data transfer allowance, but there are some important distinctions to be aware of:

1. For VPC peering connections:
- Data transfer that stays within the same Availability Zone (AZ) is free and does not count towards the allowance.
- Data transfer that crosses Availability Zones within the same region is charged at standard in-region data transfer rates.
- Data transfer across regions (for inter-region VPC peering) is charged at standard inter-region data transfer rates.

Regarding Lightsail instances specifically, the information provided doesn't explicitly mention Lightsail. However, based on general AWS networking principles:

2. For data transfer into a Lightsail instance using its private IP address:
- This would likely follow similar rules to VPC peering. If the data transfer is within the same AZ, it would likely be free. If it crosses AZs or regions, it would likely count towards the data transfer allowance.

3. For data transfer out from a Lightsail instance to another resource's private IP address:
- Again, this would likely follow similar principles. Data transfer within the same AZ would likely be free, while transfers across AZs or regions would likely count towards the allowance.

It's important to note that these are general principles based on how AWS typically handles data transfer charges. For the most accurate and up-to-date information specific to Lightsail, it would be best to consult the Lightsail documentation or contact AWS support directly.
  

Sources
VPC peering - Building a Scalable and Secure Multi-VPC AWS Network Infrastructure
What is VPC peering? - Amazon Virtual Private Cloud

Any outbound data transfer does incur cost.

Same goes with cross-regional data transfer on AWS.