First of all, you don't need to yell at her when she comes back. That will make her not wan't to came back. When she comes back (if she comes back when told) then praise her. SHe will eventually get used to the method. Daisy obviously thinks that the UPS truck is some kind of predator. She is simply protecting the property. If possible, next time the UPS, Fed-Ex, or any other truck that comes by, give them a few dog treats so the next time they come by so that every time they come they can stop and give her a treat and some praise. This method will teach her that trucks are not a bad thing. I hope I have helped.

Rachel