I have transported 2 doz beer in my Brompton shopping basket; and similarly heavily loaded during shopping trips. The problem is that the basket itself flexes. The internal frame of course does not have diagonal bracing say from front right to rear left; so the load tends to shimmy from side to side, pulling the steering in the opposite direction. So far from stabilising it, it renders it almost completely unridable. I have to go slow and try to compensate for the shimmy by holding the handlebar very loosely so that the loose grip tends to damp the shimmy. I don't know how heavy messenger loads might be but if they are in that range of say 10kg, then you might have problems.

I think a far superior solution on a Brompton (or ANY small-wheeler) folding bike is this:







The pannier is hooked on a aluminium tube tied to the seat rails with cable ties. At the bottom, a quick-release clamp holds it to the seatpost. That cam from a quick-release bottle cage bought cheaply from China. The bigger pannier fits equally well. The pannier also does no even have to be closed, making carrying very bulky object possible.

The pannier is simple to remove, takes about 5s, and also does not interfere with folding. After removing the pannier, there are no ugly fixings left on the bike, just a discreet tube at the saddle.