As an apartment dweller, whenever I see the option of using USPS for an order I will use them over UPS or Fedex. Although I only have one overly negative experience with UPS, the small frustrations of missing a delivery because I wasn't there to receive it happen nearly every time. Since I live in an apartment complex, they refuse to leave packages on my doorstep, which is fine because I've had a package stolen when the UPS man left a box in front of my door before. But, I inevitably have to have the package held for will-call at the nearest UPS center so that I can pick it up myself. Otherwise, you can bet they will attempt to deliver and fail at around the same time the next day since you will probably still not be home.
As far as USPS is concerned, if I think that the package I'm ordering will fit in my mailbox or in the larger package mailboxes, I'll pick USPS every time. If I lived in a house (in a good neighborhood) I would not be as biased towards them, because I've seen where Fedex and UPS commonly leave packages tucked away near the door somewhere unless it requires a signature.