Sorry, I Beg Your Pardon
expression
Remember 御免? This is a polite way to say it. It means sorry and I beg your pardon.
ごめんなさい
The reading comes from 御免. The addition is all kana.
御免なさい。このＴシャツのMサイズは売り切れちゃったんです。
I’m sorry. We’re sold out of medium sizes for this T-shirt.