IMO, it would be very difficult to ship through a package store and not get caught. I take fedex and ups packages to a package store a couple of times a month. I print the labels online. Even if the bomber uses a stolen credit card, LE will be able to track the device from which the label was purchased. If he goes into the store and pays cash, he's going to have to interact with an employee for several minutes. Even if he's wearing a disguise, the clerk would hear his voice and possibly notice other things about him. Plus its very likely the store has security cameras. As to scanning, when I bring a package into the store, I get a receipt with a scan that the package was accepted. Then it's scanned again when the driver picks it up, when it reaches the processing facility, when it leaves the processing facility....every step along the way.
I keep going back to the fact that there's a relationship between the first two victims. If the victims were chosen randomly, that would be a pretty big coincidence. If not, how does the bomber know these people?