Yeah, the whole stamps thing is really confusing.
Why put stamps on it if using a courier?
Did they change their mind after putting the stamps on? Did they go to the post office, and were told they needed more postage, and they left?
Are they just stupid and didn't realize packages are post marked?
Did they deliver the packages themselves? Paying a courier would really make it easy to find them.
Must have been put stamps on Brennan's and changed mind, and delivered it themselves in disguise.