Copper

Coppers are the native currency used by amphibians all over Amphibia.

The standard unit of their currency system seems to be the minted "copper" coin, which is the most commonly mentioned. Other denominations are implied to exist but have rarely been mentioned: in "Plantar's Last Stand", Mrs. Croaker mentions that she's been saving a "gold farthing" coin for something special (and it is apparently worth 20 coppers).

Appearance
Each coin has two distinct sides. One side depicts an unknown, unidentified Amphibian person of interest, possibly a lawmaker or ruler, and the other side simply shows the three-pronged symbol/logo of Amphibia.

Other payment methods
Depending on the technological advancement of a specific Amphibian area, some vendors/shops/stores may accept a primitive form of a credit card. This technology is primarily reserved for use in Newtopia, though.

One such example is the Royal Credit Card. It has unlimited funds on it and can purchase anything anywhere where credit cards are accepted in Amphibia. It is extremely rare and coveted as well, as the only person who has possession of it is the King of Amphibia himself, to which he alone can give it to a party or person at his own discretion.

Trivia

 * In "Wax Museum", Curator Ponds announces to Anne that the CD player Skip Man from Earth costs a million coppers.
 * The Royal Credit Card has an almost unlimited amount of money to spend, but not on cash-only products.
 * The cost of each food on the menu in the episode "Lost in Newtopia" also shows the Amphibian language on the menu.