If you’re the type to be disappointed by what’s under the Christmas tree, this year you may find yourself even more dismayed by what’s not under the tree. Millions of gifts may arrive late, as the U.S. Postal Service grapples with an unprecedented volume of packages from people shopping online, instead of at stores, and shipping holiday gifts instead of bringing them in person.

“We’re really busy. It’s been unbearable,” sighed Manny Huenchunir, a Postal Service truck driver who was unloading and reloading crates of mail at a post office outside Boston on Tuesday night. The main Boston processing center, he says, is overwhelmed.

“The warehouse is packed,” Huenchunir said. “Boxes from wall to wall and trucks waiting to get into dock for like three hours, waiting in line just to unload.”