A Letter of Credit is an item that was featured in the 3.5 edition's Eberron Campaign Setting on page 122. Letters of credit are issued to those who have funds deposited in a House Kundarak bank. With a letter of credit an individual can withdraw their funds at any other Kundarak bank branch. The item itself features an arcane mark created by the gnomes of House Sivis making it near impossible to forge. House Kundarak does not charge a fee for the issuing of a letter of credit. Alternatively a character can rent a safe-deposit vault.
The letter of credit does not appear as an item in the 4th edition of Eberron though, it is mentioned in the Eberron Campaign Guide on page 228 under Notaries Guild.