Hi @biswas, I’ll start by saying this looks consistent with your Redis experiment, and you’re not being incorrectly billed.
This invoice line is confusing! We are aware of this and there are improvements in the pipeline.
That 8120 is your amount of additional RAM multiplied by the number of seconds you had it provisioned. The units are funny here (the presentation, not the calculation). Extra RAM is billed by the second.
Say you used an extra 256MB of RAM for just over 9 hours (0.25GB x 32480s = 8120 GB*s). At 1.93E-6 bucks per GB, per second, times 8120GB*s, we get $0.016, rounded up to the nearest penny: $0.02.
If you had run the extra 256MB RAM for a whole 31-day month the RAM would have cost you about $1.30. 