If the item is listed in Yen in the UN Contract Catalog – does it convert the currency at some point in the Shopping Cart to reflect the cost impact for budget, or must one do this manually outside Umoja?

When calling off a contract in the catalog created in a non-USD currency, the currency of the contract will default to the SC. If you want to view the overall USD equivalent of the SC, you can view this at the bottom right hand corner (below the line items - you may need to scroll to the right).