As a partner, your tenant may ask you why they are seeing two RDS charges on the partner generated bill:
The "Amazon RDS Service" line item is relative to the commitment amount for all of your RDS RI's for the month. The line "Amazon Relational Database Service" is relative to all of the other RDS costs.
You'll notice that the drilldown line items for Unused RI have "RDS" in the product name:
All of the other RDS line items have "Relational Database Service" written out:
The true cost of RDS will be the sum of the two values.