courier collected wrong parcel now i must pay for their mistakes

I have dealt with these couriers many times in the past with no problems but yesterday I booked a collection in Johannesburg was given the booking number this morning i asked for the tracking number and saw that it showed only one parcel, i questioned the sender and found he said that they had been there but my parcels had not been collected and that someone elses parcel had been collected and showing it was on its way to me. I then get told by collections that this is my fault and that i must pay extra for this???? How can this be my fault if i am nowhere near the place where they collected and that i am the receiver this is not right I didn't tell them which parcels to collect we have a new collection reference please make sure it is on its way to me today as i have to get someone to collect from big foot in east london as i live far out of town and have to get someone else to pick up for me reference is 608027 Karen