Cloud pricing term
What is inter-AZ transfer?
Inter-AZ transfer is traffic between availability zones in one cloud region.
The expensive mistake
Assuming same-region means free. AWS can charge both sides of an inter-AZ path. A chatty service split across two zones can therefore pay for the bytes leaving one resource and entering the other even when no traffic reaches the public internet.
Where it appears on your bill
Look for regional data-transfer usage types tied to both resources, or separate cross-zone and load-balancer lines.
Worked example
AWS VPC peering prices same-region cross-AZ traffic at $0.01/GB in each direction of accounting. Moving 100 GB from an instance in one AZ to an instance in another produces 100 x $0.01 + 100 x $0.01 = $2.00. That is about 65% of the dated example's $3.07 t4g.nano base compute price.
Example prices checked: