According to Jim Recap: The Errand
As Jim realizes that Cheryl is paying $7 per week to have their groceries delivered, he goes ballistic, then offers to pick things up on his way home and save the money for other things. But then at the store he realizes that he's becoming a slave Cheryl, and realizes that he needs to do something. So he and Andy quickly hatch a scheme where he will start screwing up so that he doesn't have to run the errands any more.
Yes, that's right - he doesn't want to pay to have them delivered, and he doesn't want to do it himself. Cheryl notices the problem in the logic too.
So when Jim starts screwing up, Cheryl realizes what he's up to, and says that it's alright, she has faith in him and knows that he will get it right soon. At that point, he knows that she knows, and figures that he needs to screw up something really bad. So he "forgets" to pick up his tux for the big party. He is going to go in his t-shirt and jeans.
Cheryl decides that she'll play this game, and she's going in her bathrobe and curlers. Jim ups the ante by taking off his shirt and pants. Cheryl puts her lipstick on like she is the Joker. Jim puts it on his chest and belly, drawing a face, and they walk out to the car.
As they approach the party, Cheryl finally backs down, and Jim gives her the appropriate respect for a well-played round. She almost had him, and asks if he would have gone into the party like that. He tells her he would not have. The boxers would have come off. Nice.
