It has been confirmed that the infection stays live for about 24 hours on certain surfaces and longer on others. I hope postal workers have been issued with gloves. Otherwise they most certainly are in danger from letters and parcels, if they touch them or any surface which they have touched. It also stays live in the air, so any enclosed area could have Covid-19 particles. Like everybody else who is required to continue working, I hope they are able to keep 2 metres apart. An unknown number of postal workers is already most certainly infected and could pass the infection on to other workers or the public by contaminated mail.
We are not all safe by not touching our eyes, nose and mouth before washing our hands. Anybody who goes into a public place could pick it up on their hair, clothes, etc just by sitting on a park bench or brushing against a wall.