I don't send COA with my original paintings - I write on the back the following information:
Title of the painting
original acrylic ACEO
copyright symbol/date by Sue Peeler
my handwritten signature
I figure if that doesn't establish authenticity, a printed COA won't either.
I may include a COA with any prints I do (or may not, if I write all the above info on the back of each one, including what number that print is). I may not do limited editions - in which case I WILL number each print as I print it (assuming I'm doing them 1 at a time) - I doubt if I'd ever even do 10 of a particular image over my lifetime - but I'd like people to have that much individuality in their print.
Sue