It's a total waste of time complaining to DPD (you can tell what a mess they are in when you hear the condescending message that rewards you for holding for more than 30 mins to get through) Their service is useless. Their Customer Service is worse and just insults anyone with the smallest amount of intelligence.
There is only one effective complaint and that is to complain to the sender of the parcel and if this is a supplier inform them that you will avoid buying from them if they use DPD - they will no doubt have heard this many times before.