thanks for your take on the subject. however, i should of been more clear on what services items are. the client isn't selling chairs, they are selling services that can be applied to chairs, sofas and rooms. They are a carpet cleaning service that clean; room carpets, chairs and sofas. so there are no sizes. there is a fixed unit price for each service item and associated item.
i think that would change your concept a bit. sorry for leading you down the wrong path.
Would you still go for separate tables for "main items" and "associated items" to leverage the possibility the client may want more than one sub item per main item in the future?
Would you worry about the future later and keep original design with item/sub item pair records in one table?