A medallion signature guarantee must be completed in person because it confirms your identity, signature and legal authority to transfer securities. A letter of instruction is a document that guides a designated individual or entity on how to carry out specific actions or wishes after the author's demise.A letter of credit guarantees that a bank or some other financial institution will make an intermediary payment on behalf of the customer. Use this form for specific account requests. Bank of America provides notary services free of charge in all financial centers. Schedule an appointment and learn what is needed for a notarization. How to fill out the Bank Letter of Guarantee Template? Caret Audio assisted ATM. A bank typically issues a letter of guarantee on behalf of a client who has entered into a contract to buy goods from a supplier. Please arrive early to secure a parking spot as this garage tends to fill up early.