Liam does the lip-press-eyebrow-raise that passes for a semi-polite greeting at this hour before she moves on to the delicious abomination of nature that is Slurpee, and performs the perfunctory courtesy of shoving his crossword off to the side of the counter.
"You can size that up for 20 cents," which is not really a return greeting at all - but, like his psuedo-smile, it will pass for most folks who are not desirous of intimate conversation with their 7-11 clerk. It may be the uniform which puts them off; they are a bit like bowling shirts from hell.