Parcel opened during the deliver and items missing

On the 24th August I received my parcel to my surprise the when the driver opened the back of the vehicle my box was open. Since I was at work busy with clients I couldn't check if all items were still in the box. However I took a photo while it was in the vehicle. Fee hours later when I was able to check it I realized that there were 3 items missing. I then contacted them to report the issue and was told to send an email which I did. We continued talking via email and I gave them all the info they requested. On the 31 August I got a call from their office asked to explain again and the guy asked me to send the pictures of the content that was in the box. Did that and they told me they will investigate. On the 1st September I get the response that the driver told them that I checked everything and confirm all was in order. That passed me off because why would their drive lie, if all was in order why did I take a picture of the parcel. When I responded to that I was told they are communicating with the sender to check how may items she sent. 5th Sep I followed up and they said the want the cost of the missing items and they will ask the sender to forward an invoice. 15 September I followed up and I was not asked to sender to forward an invoice and she did that. 23 Sep follow up and the response was as long as the invoice was sent to the claims department it's being sorted out. 03 October followed up I was told the communication will be with the account holder meaning the sender. I then called the office same day and they took my details and said if we were asked to send an invoice it shouldn't have taken long. The guy was going to come back to me and he didn't. 13 October called the office again and was told they can not communicate with me since I'm note the account holder and they are still busy with investigation I asked the lady to follow up and the told her that they still investigating. How can you request an invoice and them later say you are still investigating after you promised it's being sorted. Investigating since 24 August. I have all the emails and whatsapp communication from them. This is so annoying. PATHETIC SERVICE FROM THE COURIER GUY. DO NOT RECOMMEND AT ALL