That one's a round trip of about 2 miles, of which the return journey is nearly all downhill. There's a lot to be said for a supermarket at higher elevation than home. I occasionally shop at Tesco in Quinton, which is a round trip of more like 10 miles (again, with hills), though less frequently since they stopped having sensible bulk packs of orange juice.
I suppose 40-50kg is typical if I've bulk-bought liquids and tins and things (which, along with bulky things like bogroll, is what I mostly use the trailer for - normal shopping trips are done with panniers). That's about half my bodyweight, and therefore okay effort-wise (about the same as a piloting a tandem with a non-cyclist stoker - hard work but achievable) - it's stability under braking that's the limiting factor, and I try to avoid doing that sort of thing in the wet.
I've hauled somewhat more than that when taking waste to the tip, though. It's about a 5 mile round trip and involves one short but steep bit of busy road, which is highly unpleasant with a heavy trailer. If I'm going to do something like that (or requiring multiple trailer loads) then I'll use barakta's ICE trike, which has the advantage of stability, lower gears, much better braking than a DF bike and - more recently - an electric motor. Only problem there is that you're into caravan territory - you can't see past the trailer box in the mirrors, and reversing doesn't really work. Getting the trike in and out of the house single-handed and adjusting the boom is a fair amount of work, so I'm not tempted to use it for the monthly bulk shop.
