Further on the garden centre biz: I can carry compost ok; haven't tried to carry sharp sand and I need some. I reckon I can probably carry it half a bag at a time, but then my nearest garden centre is within a kilometre.
I regularly carry slabs of six Saskia water litres from Lidl along with the rest of my shopping. These weigh 9kg, and fit snugly in a Carradice pannier.
(And yes, by the way, mostly in Ireland we're in kilometres, metres, litres and kilos now, apart from the very old. We have de-imperialised.)