In a sales order with upsell, items must be added as they are not expected to automatically be populated
To use custom price, they would need to set the Block Custom Price Level= No and Allow Custom Price Level for M/S Items= Yes on the Options & Log tab to be able to use the Custom Price Level.
When selecting a Custom Price Level, user must need to manually enter the list price because will not automatically be populated since you are telling the system to use a price not sourced from the item record
NOTE: The start and end dates for upsell items must be within the contract’s term to be included in the contract. Any items that are sold for a period outside of the existing contract’s term should be tracked on a separate, new contract.