There are three stages or classes of first class mail. Letters/cards; flats/large envelopes; packages. [Bill - the word "bulk" applies to mass mailings (like when you send out 1000 items), not to bulky items. Bulky items are packages.]

Here is the starting point for figuring out which rate applies to your mailing: http://www.usps.com/send/waystosendmail/senditwithintheus/firstclassmail.htm

Here are the rates:
http://pe.usps.com/text/dmm300/ratesandfees.htm#wp1008795

The key questions are whether the letter/package is thicker than 1/4 inch, whether it is bendable or flexible, and whether it has uniform thickness. These can be confusing, and different postal employees have different interpretations. The post office had good intentions when they designed this rate system, but they made it too complicated for mailers and employees to easily understand.

Unfortunately, the authorities who set rates are already considering future rate increases for as early as May 2008. I suspect the system they have devised will also be addressed and laid out in a more comprehensible form. I have pointed out before that it can be cheaper to mail something overseas since those rates depend solely upon weight, not thickness or flexibility.

As an aside, I still have poly (plastic) #000 mailers available for sale postpaid for $5.50 for 10, $8.50 for 20, postpaid. Thread with payment info, etc.: http://www.thechipboard.com/cgi-bin/bbs521/webbbs_config.pl/noframes/read/664877