I always ask how many extras they had room for, and also, if I had extras from different breeds, ask them if they were interested in those breeds before I send eggs out. I have been known to send as many as two dozen extras to someone who bought a dozen. Just depends if they are laying hard, incubator fully, nice person, good mood, you never know with me. Normally I try to send about 25% replacement eggs. Not always possible though.
