After reading all the replies I've thought again and now I think normally I would let him use the toilet.
But if I or anyone in the house is 'shielding' or whatever it's called I would say no, sorry and give the reason. Also as Lemon said, it depends whether you've known the postie for a long time. Ours is very good, though there have been regular replacements recently.
Last weekend, in Rutland, the first statue in Britain of the late Elizabeth II was unveiled.