Poor service delivery

I ordered an Orthofit arm sling on the 17th March only realized after tracking my order it was a wrong size. Sent a message in the morning at 09:44 to kindly change the size from medium to large as the order was still at the warehouse.

They responded "Hi Nonhlanhla, unfortunately, we are unable to cancel the order at this point as the warehouse team has already processed your order. You are more than welcome to decline the delivery or log a return for the smaller sized version and request to have the large sent instead." which I responded "Hi I just need the size changed from medium to large. The stock is still in your warehouse to be processed tomorrow for delivery on the 22nd. If I cancel the order or decline delivery it will mean delay on the delivery. I was referred by a friend who spoke highly of your service but I'm beginning to doubt.

As it will be processed for delivery on the 19th is there a number I can call correcting my mistake on the size ordered. I desperately need this item like yesterday so further delays of declining and cancellations are not an option for me. Please assist with other ways of rectifying this without already given options.

Thanking you for your response and explanation." after tracking my order I realised it was shipped at 11:29 but they claim system would not allow them to change. They could not allow me a number to call rectify this.

Suffering from a pinched nerve in C4-C5 and desperate need of a comfortable arm sling only to find I will get an uncomfortable one probably won't even fit... Told to decline order then wait again for the exchange. Truly dissapointed with their service as a first time buyer.