Difficult comma question?

Hi. I wonder what would be the key to deciding whether a comma is needed in this type of sentence situation. I think (if I am not mistaken) the decision is normally reached at the time of a person's writing and sometimes done in an impromptu fashion, by which I mean that it is done by sort of an instinct (if I correctly wrote what I had in my mind). I also think (I could very well be mistaken) that there might be a difference in perception as to whether a comma is properly used in this type of situations between the writer and reader (eventhough the whole context is understood). What do you think?

He start off to the mall to shop for a latest-fashioned shirt, with a large used shopping bag in the trunk of his car. -- Putting a comma after the word "shirt" seems to make the part underlined part secondary in relevance. (I am not sure I wrote my comment part correctly to reflect what I wanted to say. I tried, though.)