What should I do if the stuff in my package arrives damaged?

I had buyee ship out an envelope containing figure parts a week ago (the envelope was what the seller shipped the item in). They shipped it out through ECMS. just checked the tracking. It says that UniUni has accepted my package for final delivery. They will deliver my goods either tomorrow or Monday. I have a strong suspicion that the envelope, including the figure parts, got damaged during international transit. So what should I do when I receive the package and confirm that there's damage?