No notification slips for international parcels

For the past two years 90% of my international orders have gone missing. Two of them have been returned to sender with 'address unknown' or 'abandoned at post office'. Yet our local depot claims that they send out notifications. I receive other mail in my post box but so far next to no notification slips. Not first, second or third warning even – which suggests these parcels aren't even reaching the local depot. Some of the parcels were even labelled with a phone number – and I didn't even receive an SMS. I double check my address details, every time I fill in to receive parcels, and if local courier companies can get it right, what the hell is wrong with the post office? I've lost thousands of rands worth of goods. What can I do? Who will help me?