Page 146
This part was puzzling for me as well but I think I got it figured out (could be completely wrong though 
)
- When googling for the phrase 詰めの甘い you get lots of results for 詰めが甘い which seems to be an expression which kinda means “failing right at the end” or “failing to see things through”.
- Entry on weblio.jp for 詰めが甘い
- Entry on the weblio.jp thesaurus for 詰めの甘い
- An article from a japanese lifestyle/work culture site explaining 詰めが甘い (only skimmed it with the help of “translate page” in chrome
)
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I think 上品 here is not “high-quality goods” but rather the “elegance” we already encountered in chapter 1, which I myself asked about.
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We need to pay attention to what 山田 says before the speech bubble in question:「途中で止めたってわかるっての」which refers to the previous page where 佐々木 barely managed to stop himself from revealing that he uses his “cigarette pack oracle” to divine if he will be able to see 山田 at the cash register on a given day.
So to sum it up the meaning is probably something along the lines of: “You (佐々木) messed up being cool/suave right at the end (of helping me choose)”.