Leader may well know if that's possible, and how to do it if you don't know yourself and ask nicely 😉

That information can be edited on Buy-It-Now listings and listings that have no bids.  Once a bid is placed that info can not be edited or added to.

I just looked at the Sell-Your-Item page and it does say "Additional checkout information" and not "Additional payment instructions", which may give the impression that the information would only be shown at checkout.