I’ve just had something similar to the OP from Planet x.
Got home to find my one box delivered with my stuff in (handlebars, brake levers etc) and a whole pile of other stuff including a delivery note which had nothing to do with me – this pile of stuff (clothes, note etc) was inside my box.
I’ve contacted PX who said they really appreciated me contacting them and could I please send it back (free return). It’s off back tomorrow. I did reply and say that if they were that really really really appreciative, my project bike needs a set of black midge bars :). Think that bit fell on stony ground though :)