Printing costs scale with volume, while shipping remains a flat cost. If you plan to do fulfillment yourself, I recommend renting a storage unit that has access to a loading dock. Many printers won't ship to a residential address, or will charge extra to unload the books for you.

You can buy padded envelopes in bulk from Chit Chats or Stallion. Or uLine for cardboard mailers, though they have deeply unfortunate politics.