Unfortunately @whateverdadhas , even though item 257078075587 is a postcard, it was not listed in an ESE-eligible category. You cannot purchase a label for shipping it in a plain envelope.
Why did you list it in this category and in item specifics say it's signed? It's not autographed. Don't be surprised if your buyer files a not as described claim.
Sports Mem, Cards & Fan Shop > Autographs-Original > Baseball-MLB > Postcards
You will need to either ship untracked with a postage stamp, or upgrade to package and ship tracked with Ground Adv. Note the physical packaging has to qualify as a package to ship Ground Advantage.
Find a downloadable ESE category list here.
Find info on package types here.
