In many cases differences are expected between the partner and customer level. This article explains some of these differences.
A great place to start if you suspect a discrepancy between the partner and customer level is the Partner Cost History report: https://apps.cloudhealthtech.com/olap_reports/partner/cost_history
Using this report, you're able to compare the customer 'Cost ($)' against the 'Partner Cost ($)' to verify where differences are stemming from.
Some of the differences will be expected. Common examples are the following:
EC2 Compute: Due to the way that RI and Savings Plan coverage is re-applied during the Partner Generated Billing (PGB) process, you may see that EC2 Compute costs are higher at the customer level, where the customer accounts have benefited from RI float at the Partner level.
EC2 - Unused RI: At your partner level, if a customers' RI has floated and applied to another account's usage, this will show in the customer level reports as Unused RI.
RDS Compute: Due to the way that RI and Savings Plan coverage is re-applied during the Partner Generated Billing (PGB) process, you may see that RDS Compute costs are higher at the customer level, where the customer accounts have benefited from RI float at the Partner level.
RDS - Unused RI: At your partner level, if a customers' RI has floated and applied to another account's usage, this will show in the customer level reports as Unused RI.
CloudFront - Transfer: PGB will recalculate the customer level CloudFront - Transfer costs using the AWS List price. This ensures that your partner tier discounts are not passed down to the customer.
AWS Support: AWS Support charges are often recalculated using Partner Billing Rules. Please check if you've applied a Support Billing Rule to your customer.
Tax/VAT: Tax and VAT are always removed from the customer level bills.
Partner credits: Partner credits are always removed from the customer level bills.