The process server threatened to put the summons in the mailbox if Fred wouldn't take it. Legally, nobody is allowed to put something in or take something out of a personal mailbox except the owner of the mailbox or a postal employee.
The bill from the drug store shows Fred bought Pluto Water. The episode was filmed in 1972, but Pluto Water was taken off the market in 1971 when its main ingredient, lithium, became a controlled substance.
It isn't explained why Fred and Lamont never attempted to sneak out the back door of the house (through the kitchen).
Lamont tells the collection agent to make out the check to him, yet Lamont's name is never mentioned while the agent is in the house.