Once we finish and rollout full VPC support we believe the absolute majority of use cases will not incur bandwidth charges since most apps run on the same DC as their DBs.
We do not plan to charge for private network / VPC traffic.
We have not yet decided to extend the free egress public network bandwidth beyond Dec-2019 and whatever happens we will provide users with time to plan for the change (say 60 days).
If we decide to not extend it, users will incur .01/GB in all DO regions which is one of the most competitive bw fees in the industry.