Peter showed me one of those when he came by. It works basically as a localized form of time travel; it creates a temporal gate that links one point in space-time to another by working backwards. Basically, you say you need to get from Los Angeles to New York in a hurry, and you’re in the past. You can take a jet, which will get you there in a handful of hours, or you can open a wormhole in space-time from your pad in LA to an out-of-the-way alley in New York. It takes time for the matter to travel from one end of the country to the other, but the wormhole moves backwards through absolute time at the same relative time as it takes you through relative space. So, while you walk maybe a handful of steps through the warped space of the wormhole, time dilation and the backwards travel cancel each other out, and you come out the other end of the hole sometimes before you enter it on the first end.
It gave me a headache when he explained it, and I threw up after we got back from the Vatican.
So, someone has a warp-walk unit, and is using it to stalk Bert’s clients. There’s any number of ways that the thief could be aware that Bert’s running his game, and any number of ways that the thief could gum up the works on each package. Theft is just the most common method.