I believe it was back in 1997 that UPS configured a few of their 727's such that they could slide in a cabin 'module' and convert them to passenger service for the slower weekend period.

I've seen a few reports (notably on cruise critic) of folks being flown here or their to catch their cruises on UPS birds.

Not certain whether this practice continues today.

Makes good economic sense though, like everybody else they get paid when their birds are in the air, not while they sit on the ground.

Lihue1k