<prompt> Translate the following text to EnglishKwa miaka kadhaa, baadhi ya mailers wamekuwa kufuatilia wazo la kuanzisha maalum posta viwango na au masharti ya huduma kwa njia ya mazungumzo na Huduma ya Posta.  <response>: For a number of years, some mailers have pursued the idea of establishing customized postal rates andor terms of service through negotiation with the Postal Service. 