These are not flat rate. The price is exactly the same if you use UPS packaging or your own packaging. If you use your own packaging AND you also stick it in a UPS Pak, you're getting charged for the extra weight and dimensions that the Pak adds.
This is the exact opposite of Fedex Express. All Fedex Express packaging is flat rate. It is always cheaper to ship in Fedex packaging than it is to ship a box. Fedex also provides negotiated flat rates if you use their packaging. They really want you to use their packaging.
Can an account rep give me flat rate or reduced rate if I use UPS packaging? Does that even exist? If that's not possible, why would anyone ever use UPS packaging for any reason?
Edit: I know 'UPS Simple Rate' exists, but the rates are terrible and they make it clear that negotiated rates do not exist. Are negotiated rates on UPS Simple Rate possible?