The below is just from my experience and how I was taught. I am unsure if your question was for ACEO only or Traditional canvas or both, so included both ways of handling them.

Smaller non impasto, thin layered oil paintings can be dry enough to ship with 7 to 14 days time. ACEO's should be sealed with clear varnish and placed in a clear archival acid-free sleeve and a clear rigid card holder for shipping.

For traditional oil on canvas the following apply.

I wouldn't suggest removing it from the frame. The buyer can have a lot of difficulty restretching it. Rolling a canvas works best if it was never on stretchers to begin with.

Be sure that it is dry before you ship it. It can take up to 6 months for a painting to considered dry enough for varnishing/shipping. Even though an oil painting is dry to the touch, it can still be wet in the lower layers and vulnerable to damage. You also do not want to varnish an oil painting for at least 6 months (if you sell if sooner include information to the buyer that they will need to have it varnished to preserve it). If you sell it locally (item isn't being shipped) offer to varnish it for the customer in 6 months time.

Use the bubble wrap method. Remember to NOT let the bubble wrap touch the painted surface. There can be a chemical reaction and destroy the surface of the paint. Wrap your painting loosely in paper (like newsprint) and then wrap stiff cardboard around that to keep it from getting any puncture damage in the shipping. The next layer should be bubble wrapped to keep it all from getting crushed or double box it. Or, places like UPS can package it for you safely.

Hope this helps.