Why we only offer set quantity NOT drop buying any more

Take the Irene's case as an example.
She bought just 1 pcs of hard rubber feet from us, after some time, she came to us and said this feet splitted.
But it was very likely this splitted one was not from us, but other 3 old ones.
However, we still sent her 4pcs later for free to solve her problems. But from it on,we stopped offering drop buying quantity, but only set.