Unless the buyers have permitted for a parcel to be safe dropped then the postie should not safe drop it.

Where would the postie safe drop a parcel? in the letter box? at the front door where passers by can see it?

Or in your back yard where the postie isn't allowed to enter?

Unless permission is given then safe dropping isn't allowed.

A buyer could easily say they never received the parcel even if the postie took a photo of it,who knows if the postie decided to take it with them,taking a photo as proof doesn't cut the cake in my opinion if a signiture was required.