Ours did that ALL the time until I complained to our local PO. His excuse was that we get too many small parcels and he can't fit them on his bike. Which is absolute rubbish - now that posties aren't delivering as many letters and more small parcels, their bikes are being modified so they can carry more parcels.

He now delivers our parcels, but if there are too many to fit in the letterbox, he chucks them onto our front verandah. And if we get anything damaged, that will be another complaint to AP.