The site could discourage $5 buyers, meaning psychologically letting them know they are not getting full benefits like all those other buyers, in subtle ways by making them feel like second class citizens somehow. What I mean is making them feel bad for only spending $5, so that they get at least one extra.
This could be done by automatically giving sellers an extra day to deliver on any $5 order.
Or not allowing the $5 buyer to leave less than 5 star feedback.
Or sending a pop up message “89% of buyers get at least one extra with this gig”
I’m just throwing these ideas out there. It seems like there could be a way the site could automatically encourage buyers to spend more than just a $5 gig.