查了一些英英網路字典，好像真的沒有人把 "not at al" 當作不用客氣的意思，

看起來 "at al" l都是用在偏向否定時的對話，像這個：
http://dictionary.reference.com/search?q=at%20all
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at all

adv : in the slightest degree or in any respect;
"Are you at all interested? No, not at all";
"was not in the least unfriendly" [syn: in the least, the least bit]
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"not at all" 雖然是 "一點也不" 的意思，但用在回答"Thank you."似乎是有點奇怪，

起碼好像google不到這種用法的...

我本來也覺得是不用客氣啊~
真迷惑。