This character is one I thought of as like my third ever character but that version of them wasn't really working. I think I've got them now and I really like this character.
Fleetfoot is an interesting item endgame build wise because it's a tier 2 you just never want to sell. Well here's the interesting upgrade. It can also double as a magic carpet alternative.
Have two alternate versions of the tier 4.
As far as how upgrading an ability past what this unlocks would go, two options. 1. You can't. If you want to upgrade it more you have to either buy the upgraded version of this item or sell it to use your own ability points.
2. You can keep upgrading...
Ammo mastery would upgrade out of lucky Bullets, Active Reload, and Specialist Ammo. It would therefore cost 0 if you purchase the three of them as they alone would total up to 6400.
Edit: the Ammo mastery description on the red bullets should read the same as the specialist ammo.