Do postnasal drips cause bad breath?

Yes. Especially if post nasal drip is caused by sinus infection.
Many causes. There are many causes of bad breath and post nasal drips are only one of them. Here is a website for more possible causes and information. http://smilesapartcosmeticdentist.com/fresh-breath-control.htm the most common cause and one of the easiest to treat is poor oral hygiene. Confirm the cause first and then see a dds or ENT for advice.