Inaccurate discrepancies claimed

I am very disappointed by Express Online Couriers - The Courier Guy. I've sent a parcel with them. They have just got back to me saying there are discrepancies with my weight and dimensions. The dimensions are very far off to what I have recorded. They are in the position to do that to us because they have the parcel in their possession and can request further money from us. This is very unfair and its not true. I am surely not making use of their services again. The "discrepancy" they record are almost doubled of what I have paid already. This is a very good strategy for them extract more money from us.