Surely most drugs could be sold in small enough envelopes that you don't need parcel postage. I know when I was using DNMs most packages were a simple large letter stamp stamp which covers 35.3cm x 25cm x 2.5cm and a maximum weight of 750g. So I doubt dark net drug orders would have been the trigger for this sort of change.

As long as Congress thinks drugs are shipped in giant parcels, it doesn't matter how drugs are actually shipped.