I have such a variety of shapes and sizes that there’s no one standard way of packaging for me lol.
But the majority of what I post is paintings, from tiny ACEOs (2.5 x 3.5 inches and Inchies (1 x 1") up to very large canvases for custom commissions. I always keep a stock of board backed envelopes in, along with cellophane wrappers, so that even if the envelope gets damp the painting is still safe inside the cello bag.
My wooden pet signs get bubble wrapped, especially to protect the corners against knocks, and the wrapped in a plastic envelops (my colours of choice are lime green or bright pink!).
I have the Moo mini cards to pop in, and did print some compliment slips (which I have mislaid). When I was less busy I would pop in a hand written card as a thank you - I have to admit that I’m so busy now that I tend to miss that out (I’ve always chatted to the person on line for commission work anyway, and said thank you for the order at that point).
I get really annoyed with myself if I come back from the PO and found the business cards that I’ve got out to be included in the package still on the table - I get so wrapped up in making sure everything is safe and secure that I do sometimes forget to put them in!
I also like to pop in a wee free item - bookmark, card or similar - but have had so much to post lately that I need to make some more freebie bits and bobs as I’ve run out!
I always make sure I use my printed return address labels on both the inside wrapping and on the outside too - just in case the outside one gets damaged - better safe than sorry