I chose this example of the importance of correct comma usage partly because the article I found it in was interesting.
Here’s the rule:
Words between two commas are an aside, and can be removed from the sentence; the sentence still stands.
Here’s the example. Think about the meaning if you take out that second comma.
In June, Amazon bought online pharmacy PillPack, a startup that ships medication directly to customers, for $1 billion.
See the difference? As the sentence stands, they bought PillPack for a billion dollars. If you take that comma out, you have PillPack shipping meds for a billion. More than I’d want to pay for some pills!
The article in the link is interesting—Amazon is rather diversified.