The most common type of bulk mail is called “Standard Class”. (Years ago it was called “3rd Class”.) Most advertising mail, newsletters, etc. are sent using Standard Class. To use Bulk Mail, your mailpieces must be essentially identical in size, weight, and appearance, without any personal information.
That means you can’t scribble sticky notes to your friends on various pieces. It also means that you can’t send invoices, statements, and other individual information using Standard Bulk Mail. But you can send form letters on which you personalize the name and address, so long as the rest of the letter is the same for everyone.