] There's a discussion of the legalities of franking in the archives here.

Thanks, Caroline. The book I'm reading (Robinson, H. The British Post Office; A History, 1948) also has quite a bit on franking. Apparently abuse of the privilege was widespread. According to Robinson, numerous measures were instituted over the years to curb the abuse, but they were pretty much a failure (hardly surprising, since the people making the laws were the same ones breaking them).
Robinson makes several references to people sending letters & parcels by "other conveyances," but doesn't specify what these might be, other than sending things by friends who were travelling (as Mary Musgrove sent her letter to Anne in Bath via the Crofts). My knowledgeable colleague also suggests that people whose jobs involved travelling would also carry letters and parcels for a small fee (though as far as I can tell this would have been illegal, too).