Item Sale – Delivery due date 18th December, customer requested return 15th December due to ‘Missed estimated delivery date’ auto authorised by amazon?

I have had a customer purchase on Saturday 14th at 10am and item via standard delivery with an expected date of Weds 18th December.

Today (15th December) I have a return request based on ‘Missed estimated delivery date’ to which Amazon ‘Auto-authorised’ the return. I only shipped it yesterday, so now paying shipping to the customer and back.

Looking for some advice, I am new to amazon selling. Is this normal practice with amazon or is it likely I have setup something wrong here. I don’t have Sunday as a shipping date, and have an 8am cutoff time on Saturday as based on this it it would not have even been required to be shipped yet.