What process should be used to use items which should not be used in Sales order line but shippedhi

Hello All,

My current client has an business requirement, wherein there are 2 types of items , One is Billable consigned items and another one is Ancillary items which are non billable items.

When we create an Sales Order the client wants to see only the Consigned items to eb part of sales order lines , they want the ancillary items to be shipped with these Consigned items but should not be used in the sales order lines. Can you please suggest how this can be accomplished?

Few ideas which came to mind , shall we create BOM for this purpose? If so can we just use standalone BOM model in the sales order line with out using ATO or PTO types?