At 1m long, your items were not eligible for Simple Delivery and the postage is stated as 'other courier'.
When you set up the postage, it looks as if you set up 'no additional postage cost' if the buyer bought more than one item.
This is not a Simple Delivery issue. It looks as if you made an error when you listed. You will have to check your original listing to see if this was the case.
If it was your mistake, then you should really honour the sale and the postage cost or try and sort something out with your buyer.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)