‘Much’ and ‘sir’ mean things that are entirely different, yet “thank you much” will serve for “thank you, sir” where a simple “thank you” would come up short –the mere “thank you” is in this case lacking a needed syllable in the poetic meter of ordinary discourse.
Similarly, I will never answer a “good morning” with “good morning” but always “hi” or “‘mornin'” (though I would never initiate a salutation in the morning with those words.) If I do ever respond to a ‘good morning’ with a full on ‘good morning’ I tend to pronounce it as a kind of exaggerated dactyl with the ‘good’ taking twice as long as the ‘morning.’ ‘Good morning,’ they say. ‘GOOOOOD mrnn,” I reply. All of this is in keeping with the prosody of the everyday.
Something I’m frequently guilty of is completely skipping a whole line at the tail-end of the customer exchange. I say thank you, they say thank you, and then, according to this prosody, I’m supposed to say, have a good one or have a nice day, and they leave; but these expressions seeming often too emptily commercial to me, I will simply say nothing, and the gaping hole that opens up, the social void, creates a feeling so awkward and imbalanced that the customer themselves will supply hesitantly the phrase.
The Thank you trap. A related phenomenon is how thick and fast the thank yous can start falling around the register if customers and attendant both don’t carefully husband their use. Hand over the coffee –thank you says the customer, thank you rejoins the attendant –hand over the credit card –thank you– thank you again– hand over pen and credit card slip –etc. etc– in short, every trival exchanging having come to required an expression of gratitude.