If you want to complain you will need to call customer services but don't expect them to call you back. Write everything down, including names of the people you spoke to, dates and times and what was said by you and them.
I had a two year battle with Royal Mail because of one truly terrible employee of theirs and finally had to go to the ombudsman to get anything done.
Long story a little bit shorter, the postwoman suddenly decided to start leaving our post with DH's relatives, who lived at the other end of our street but who we saw almost nothing of. Not just parcels but letters etc. She didn't leave a card to tell us where our post was either so we would get things weeks after they were 'delivered'.
Asking her to stop and either post them through our door or take things back to the post office if we were out didn't work. We tried three times before we decided to complain officially.
She then left a parcel of electrical items on our front path because we were out. We were away for five days and it rained the entire time. The parcel was ruined. This was six months into the issue.
We complained again. She then counter complained that our dog bit her through the front door. Dog was inside, she was outside, and she insisted she didn't put her hand inside the letter box even a tiny bit. Dog is tiny and I will never know how she managed to scale to door and get her mouth through the letter box to bite someone who was completely outside. She also didn't notice she had been bitten until she was in a neighbours garden and she didn't need any kind of medical treatment. Neighbour spoke to us afterwards and said she found blood on her own path and thinks Postwoman hurt herself in her garden but blamed us to get back at us for complaining.
We put up an external box for the post, at which point she stopped all parcel deliveries to our house and forced us to go and collect them. We said we would take the box down and she said she wouldn't deliver anything in that case.
A year of fighting that went by before we finally exhausted all efforts of customer services and complained to the ombudsman. They ruled in our favour on submission of the evidence (which included a picture of the dog in a pink hoodie my neice bought her for Christmas, proping up newborn LO in blue hoodie that matched - expecting SS and RSPCA tomorrow for that but it was only for one photo lol) and pictures of the letter box etc. They agreed that a dog could not possibly get to her through the front door and that either she put her hand in or she lied.
It turned out that she had raised a complaint against every house on the street that had a dog so that she didn't have to knock on the door or go in the garden. She wasn't scared of them either, she kept dogs herself.
Parcel deliveries were supposed to be resumed but she dragged the problem out a few more weeks.
And that long story has the point that during all of this fight, every single customer services employee I spoke to during that time told me that they cannot just leave parcels outside your property even if you ask them to. You have to designate a safe place and have the person sending you the parcel put a sticker on it to tell the postman that they and not you give permission to leave it somewhere else. Even if you have paid for the item and the postage the royal mail don't accept that you are their customer because you didn't physically go and buy the stamp, so it's not your permission that matters.
Your postman probably thought he was doing you a favour. Ours puts things in or behind our green box but I asked him not to because our neighbour often puts our box out when he does his own, so now he hides things elsewhere or leaves them next door.