Definitely complain.
Track down the delivery office that your postie works from and write to the manager, keeping a copy of your letter. If you don't get a satisfactory answer then keep complaining.
Royal Mail now have competition in the market place and they have to do the job properly, so your complaint should be taken seriously by the management. Maybe not so by the actual postie, who may try to get his own back for getting a b******king - your letters may be the ones that get soaked when it is raining! But keep complaining and management will get the message that this postie is not fit for the job.
I was a postie for a few years and sadly there are lazy postmen. It's a hang over from the days of "job and finish", ie when the walk was delivered then the postie could go home (or to his afternoon job), so it was an incentive to take short cuts whenever possible, and just dropping parcels without knocking the door is one of them.