It would be far more simple to arrest on a local charge such as receiving medication without a prescription. If the envelope did not have the proper prescription label that would be enough. There might be DEA involvement but they would probably be part of a local narcotics task force. I do agree US atty's and a federal judge might not have the time to make an example out of a 25 pill order, but local LE does work through local intra agency groups to direct the flow in a case like this (assuming there even is one).
In all likelyhood the package was stolen or lost.
Take care. Billylll