Poor returns policy

On Monday 30 November I tried to return two window hinges. I had mistakenly bought the one aluminum windows and not wood. These are Builders branded products, yet they refused to pass me a credit for one of the two because I had only kept the latest slip. No slip, no credit! Even though it is a Builders branded product and packaging was still intact, I had not opened it. I was not asking for my money back, just a credit so I could get the correct product. Branch manager not available. I think this is shockingly poor service!