Commas are usually problematic for many writers. When do you put one in? When do you leave them out? When I’m reading, nothing ruins a story more than bad grammar and improper comma usage is at the top of my list.
Commas are optional but mostly unnecessary after the second to last item in a series of items such as: I went to the store and bought lettuce, tomatoes, onions and peppers. The conjunction gives enough of a pause that the comma is unnecessary.
Speaking of pauses, I’ve heard many an editor say that a comma should be used where there’s a natural pause regardless of grammar. I find that a thoroughly uneducated statement. If pauses don’t fall with grammar, then the sentence is written incorrectly.
Commas used to join two independent clauses seem to be even more problematic even for grammar experts. I’ve perused several grammar websites and there is little to no consensus on both the presence and location of commas. Some sites place them before the conjunction, some after, and some say no comma.
I generally do not use commas when connecting two independent clauses. I feel the conjunction gives enough of pause that an additional one is unnecessary. A comma means a significant pause, which, in most cases, disrupts the flow of reading.