Unhappy Loyal Customer

I have on two or three occasions ordered items which I was told later that they are out of stock. The issue is that how do you allow customers to continue placing orders when the item is out of stock. My order of 26 August 2021 was delivered without Multi-purpose foam cleaners and there was no communication to that effect until I wrote an e mail on 11 September 2021 to find out about those items. I got a reply on 14 September that they are out of stock and to offer me store credit., which I responded on the same day that this credit thing is problematic because when you use them you have to pay delivery costs again. To date I have not received the credit store voucher.