To manage this, I have one of those keypad locks. I change the code between each stay, so each guest has their own code. This also prevents re-entry from former guests. I tend to send the code to my guests, by message, about 24 hours before check in. If I have more than one guest in a week, I will make up the codes ahead of time, and only change it so that it works for the appropriate guest just before their stay.
It is likely that your guests will be able to see the lockbox code the moment they book.