A coin to pay Charon for passage, usually an obolus or danake, was sometimes placed in or on the mouth of a dead person.

What is Charon's fee?

Those who passed away would have to cross the rivers Styx and Acheron to reach the underworld, and Charon would take them on this journey. His fee for carrying the dead across the rivers to the underworld was a single coin, usually an obolus or danake. This coin was placed in the mouth of the deceased prior to burial.

Why do you pay Charon?

Coins (specifically a type called an obol or obolos) were left on the body or placed in the mouths of the dead. The dead give Charon the coin, which shows they have had proper funeral rites and therefore deserve to be transported to Hades.