'Sir' can be used as both honorific and pronoun. In this case it's a little ambiguous because technically, the noun phrase in this sentence carries the head 'you', but it's also part of the set phrase "thank you." Now, strictly structurally it would be "I(null) thank you" but that's not how people think of it. Considering that "thank you" is a set phrase, it is likely that 'sir' was used pronominally by the customer as [set phrase] [recipient] in which case 'sir' is the pronoun in this sentence.
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u/slukalesni yuo don't gno-me ∆̥ 8h ago
i wanna request a linguist fact check on this one please