*note: some of these links are my affiliate links. That means that it's a win-win for the both of us: you get great deals and I earn a small commission (at no cost to you!) As an Amazon Associate I earn from qualifying purchases.
Bags and bows offers packaging in a wide variety of colors and are highly likely to have one that coordinates with your brand. They stock everything from boxes to bags to tissue paper: basically every kind of packaging you can imagine.
Mailers USA (quickly!) ship for free to the contiguous US and have a nice variety of colors to coordinate with your branded packaging. They are a great source for padded mailers and other shipping supplies.
Get custom stickers printed with your logo or any other images you prefer. These are especially useful for customizing your packaging at a reasonable cost. Click through here for a $10 credit on your first order.
You can order Priority Mail shipping supplies for free from the USPS website, and they ship them (again, for free!) directly to you. There are many options here that aren't usually available at the post office, including a padded flat rate envelope (which is bigger but the same price as a small flat rate box).
Nashville Wraps is another great source for packaging and shipping supplies. They have a wide variety of colors and sizes for internal packaging, bags and shipping boxes. Shipping can be on the high side, but if you plan right and keep your eye out you can stock up and get discounted or free shipping when you buy in bulk.
Vistaprint is a quick, easy, reliable place to get your business printables made. Vistaprint offers business cards, postcards, notepads (for email signup lists!) and much more. They frequently have sales running, so make sure you search for a code before you check out.