What do you do with all those extra buttons that come with garments? I have a container of them at home, and can’t recall ever needing to use one as a replacement. Maybe it makes sense to include them if the buttons are a key feature of the apparel or if they are particularly distinctive, but I think a lot of resources are wasted by including generic buttons with almost everything.
Think about other items that would be better served by having a spare included. Pens could come with extra refills. Gizmos could have a space for extra batteries to be on hand, allowing you to do a quick replacement when the originals ran out. Ornaments could come with hooks. Expensive shoes could have extra heel pads that you could snap on at home when the others wore down. Dog collars could have a compartment for a spare “pooh bag.”
It seems to me that there are many items more frequently needed than a replacement button. Maybe you can anticipate when your clientele will need “another” and plan in advance to include it. But to automatically make an “extra” available seems to be a vestige of the past when replacements were not easily located and resources were a plenty.
Take a good look at what you provide in duplicate and pilot an experiment instead. Make it incredibly easy to ask for another, and see how many actually do. My guess is that requests will happen about as often as you have sewn on your spare buttons.