They will tell you what you need to know. For large quantities, third class bulk mail is far cheaper than first class. Therefore, unless time absolutely does not permit, use bulk rate for large quantities. If you plan to mail more than 1000 pieces during your campaign, third class will save you money, assuming you mail in quantities of 200 or more, as required by the post office. The least idiotic is the requirement that your mailing be separated by zipcode sections, and that each section be zipcode sequenced.
Post offices differ enormously in the manner in which they treat bulk mail.
It always helps to establish a cordial, personal relationship with the Bulk Mail Division. Never mail third class unless you are sure you know exactly what you're doing. It's worth it to find out. The best time to use direct mail is the few days before the election. Unfortunately, every other candidate will also be mailing at that time.