Custom Price Book rules are not modifying Marketplace LineItems.
CloudHealth
Modifying AWS Marketplace billing entity line items are currently not supported by custom price books.
Workaround is to add a negative refund amount as a Custom line item to the Marketplace cost from customer tenant.
*Login to the CloudHealth application --> Switch into customer tenant --> Click on 'Setup' --> 'Custom Line Item Rules' --> On top right corner click on 'New Rule' & select 'Add new line Item'.
-> Enter the Name for the Custom line item --> Select the Cloud --> Choose Exclude & select the accounts from drop down.
-->From Line Item Details select : Credit: decreases total cost.
-->Frequency : One time
--Amount: Manually entered cost.
--Enter the Cost --> Select the 'Service/Service Item Spend' --> Exclude --> Amazon Marketplace and click Save.
Customer would see Marketplace cost in 2 line items after applying the custom line item in channel customer level.