Hey Chuck... regarding paypal features/upgrades...
I haven't tried this yet to see how it works, so I'll just ask you...
When someone purchases the highlighting is it automatically pinned at the top of the category list?
If not, then maybe you could create an additional paid add-on option to pin the highlighted add at the top of the results?
If so, maybe you could have an option to break it up and have 2 charges for highlighted and pinned.