I know that we don't often add a comma before a preposition, but I HAVE seen commas coming before prepositions like "despite" and "before" from time to time. However, I'm not 100% sure that the sources where I've seen these instances are reliable; moreover, I have once been told by a native speaker that a comma before these two prepositions is unnecessary. Look at this example:
Mexico’s kiwi fruit exports started at 6 million New Zealand dollars in 2010, after which it experienced a significant decline to hit a trough of 2.4 million in 2011, before recovering to end the period at 3.3 million.
I think that the comma before the preposition here creates a nice pause because, without it, it seems that the second clause is a bit too long. Furthermore, the comma separates different trends in Mexico's kiwi fruit exports and makes the sentence more comprehensible. So, can I add a comma before a preposition just because I feel that the sentence needs a pause?
P.S. One more concern about this is consistency. If adding a comma before a preposition is a matter of style, do I have to consistently do that in one essay, or just do it wherever needed?