https://portaldev.eonpayments.com/api/v01/merchants/[merchant-code]/letters                
            Represents letters definition used by the merchant for debt aging classification.
UniBill only
Note: not supported by sub-merchants (merchant accounts).
                                                                        | Method: GET | Implicit: Yes | Returns: MerchantLetters | 
| Consumes: xurl | Produces: json, xml | |
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                                        Loads letters information of the merchant 
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