Refund complaint

I bought items from RunwaySale and later I saw another item that I wanted to add to my existing order in order to avoid being charged delivery twice but the agent by the name of Geneveve advise I can't add anything to an existing order, she then advise she can cancel the order for me and re-order again and she asked if she can put the money I paid to their store credit and I said yes and she did not advise of disadvantages. And she did mention that I might not get the items after canceling but I told my self that if I do order immediately then I will find them unfortunately I did not,

Now the issue is they don't want to refund my money saying it's in their store credit that I can use anytime and I have advised I will get the items elsewhere as they don't have what I wanted and I can't wait for them to have another sale and am transferred from pillar to post even their managers service is pathetic. I told her no one advised that once the money is paid to their credit cannot be refunded but still fail to get the point. and why keep my money as they don't have any store account? Are they going to give me my money with interest as it benefits them?? They cannot force businesse where is not needed.

Please I need my money.

For assistance will highly be appreciated.