cancellation of order

I ordered a product by mistake and i called the exact minute the order was confirmed to cancel it, they said they couldn't allow it and it by the warehouse. i didn't understand how they couldn't cancel a order if it is still with them, i am very disappointed in their service and highly doubt i will ever choose their services again. I can not seem to understand how a process so simple can not be executed. i once again would like to highlight my disappointment and regret