Accept nothing they offer you without cause. But if you do them a kindness, without thought of recompense, and they offer a gift in return regardless, then it is generally safe and likely beneficial. Generally— exceptions do apply, particularly in the case of user error or suddenly getting really greedy with the gift.
Errr… this applies to the fae, but not customers, I guess. Perhaps the analogy is not perfect then.
Just let anyone scrape it all for any reason. It’s science. Let it be free.