I don't think your behavior was wrong, but it was possibly a drawn out for the magnitude of the offence - although demanding six dollars for spackle while returning a damaged bathing suit takes cheek.
At this point I think it's worth dropping the frostily polite behavior - a short time is okay, but carrying it on for weeks would be excessive. Be pleasant to them, but don't make any effort to be particularly buddy buddy, and avoiding going to any more events at their place is perfectly legitimate, as is refraining from donating any more clothes.
They've been rude, and there are consequences for that, but publicly snubbing someone should be reserved for more serious offenses.