There are further more complicated ways which basically involve buying stuff you use loads of at a low quality and buying very high quality for the rest so the quality averages out. Remember the quality is a weighted average. I've seen the opposite too, buy one big usage product at high Q and all the rest at low Q.
This tactic is known in game slang as ASQ (average supply quality), a good explanation is provided in the thread 'wildeyes leaving guide to everything', search for it though it is getting a bit dated, it'll give you the basic idea. Any search for ASQ should enlighten you, if you have the time and patience to use this strategy.