rediculous service just less then an hour after incorrect purchase price

i went in store to enquire about an electrical part for my vw citi golf. of course it needed to be ordered. I was hot and tired, it was a hectic work day and misunderstood the quoted prices for new and second hand parts. I went ahead and made payment, didn't bother looking at the printed A4 paper receipt without much information and waited for the salesman to call and order the part. in less then an hour I took a look at the slip and realized I paid a little too much then I thought, R1695 to be exact, I was however interested in paying just under 500 for used parts. I made a call to the store and explained what I just realized and was blatantly told there is no refund as the part has left its destination but only to arrive on Thursday. I am reeling in shock because it wouldn't have been an issue to cancel the order in just over an hour after being there. I have been a customer to this store before as parts are trustworthy as I have been driving vw for over 20 years/ please tell me there is way around this honest mistake