One of the bits of advice I remember seeing in the last few years is that many second hand / charity shops are overwhelmed by donations, and seeing if one can rehome stuff before donating is worth doing. Particularly if one isn't sure of the quality -- people on buy nothing and equivalent will take older/scruffier items because they know they have a use for it, while an op-shop doesn't want to try (or can't)

Also: these shops should not be a dumping ground. Not sure about where you are, but our local opshops have to pay to bin donations that aren't in good nick.