The invitation letter or Authorization code number is the 18-digit code that the Ministry of foreign affairs send to us. Receiving this code means that your visa is ready and you just need to collect it. Generally, for most nationalities, it will take around 7-10 working days and for special nationalities such as Americans, British and Canadians, it will take 2-3 months. Having this code, you can refer to Iran embassy in your country or Iran airports to receive Iran Visa Stamp.