It was good and then it wasn't

I made a sale but didn't receive all the items that I order. There was no letter or info from them to advise on the omitted item. When I contacted them I was informed that the item was out of stock and the person wanted to know if they could give me a voucher to the value of the item already paid. I've confirmed that I would accept a voucher but no voucher info was provided (this was after 3 attempts) I was satisfied with the items that I did receive. They were good quality and all in working order