You're the one keeping this topic alive - I said my piece. Just sayin'...
But since we're on the subject again: I emailed an eBay seller about a radio I'm interested in. An antique CB that's going for cheap. Priority shipping (yet again): $76. I contacted him, now its ground for $20! Large, heavy objects don't do well on Priority. Large, light objects don't either, but not quite as bad. Small and light usually, but not always, goes for a bit cheaper with ground. Where (flat rate) Priority shines is small but heavy objects. It doesn't make any sense, but small and heavy objects ship for less Priority than ground - sometimes WAY less! I suspect most of your merchandise falls into this category.
I'm selling some oil diffusers on eBay that would fit into the $15 box. They're light though, so they're going ground for $7.50 - $12-ish depending on where they end up being shipped to. If it were something heavy - like a locker or somesuch, the $15 box would be the way to go for sure!
and yes, few people get it.