These are thetypeofbooksthat aregenerallysentunderplaincover.
Thebookwassentto him underplaincover.
I would like you tocheckthefinalproofs, which I amsendingunderplaincover.
We aresending, underplaincover, aprivateinvitationto thelaunchevent.
Theinformationwassentto James by thefirm, with aletterunderprivatecover,informinghim of the project'sprogress.
See alsocovernoun(SOMETHING PLACED OVER)