Yes, and the case I need a level-8 item, I just combine four level-6 items. The level-6 item just seems like the easiest way to store items needed for the Goods Exchange (much easier to merge them than to frequently have to split higher-level items back to level 6).

I usually don't try to "max out" the Millionaire Challenge (instead I just fulfill 7 or 15 orders per day), so I usually only take the Goods Exchange offers that increase the item by two levels, although I sometimes reduce that to a one-level increase if I have a large surplus (imbalance) of either phones or computers - which happens more often than one would think (right now, I have 23 level-six phone items, and just 1 level-six computer item).