It's actually a pain in the rear trying to send an extra plant as a bonus, for me a least it is because I try to send something that I think they will like. Sort of if they bought only eyed daylilies from me I try to send another eyed daylily or if they bought a pattern I try to send another pattern. It's just so much easier to send extra fans when possible as the bonus. Some people just don't have a lot of extra plants to use for bonus plants either.
What I did for a few people was if I saw them bidding on something and didn't win it and if I had another I offered to give it as a bonus so they wouldn't have to bid on it and maybe not win it if I relisted it.
I too like to get extra fans.