per vCPU-Hour of T3 CPU Credits

Hello.

If you look at the shared bill image, you will see that you are being charged for burst credits.
T3-based instance types can burst beyond their performance when loaded.
Bursting will incur additional charges as described in the documentation below.
https://aws.amazon.com/rds/instance-types/?nc1=h_ls

While Amazon RDS T2 instances are restricted to baseline performance once the CPU Credit balance is drawn down to zero, Amazon RDS T3 instances can burst above the baseline even when its CPU Credit balance is zero. For a vast majority of database workloads where the average CPU utilization is at or below the baseline performance, the basic hourly price for t3.small covers all CPU bursts. If the instance happens to run at an average 25% CPU utilization (5% above baseline) over a period of 24 hours after its CPU Credit balance is drawn to zero, it will be charged an additional 9 cents (7.5 cents/vCPU-hour x 1 vCPU x 5% x 24 hours). See Amazon RDS Pricing for pricing details.