Here are a few simple, respectful replies you can send for Saatam:

• Jai Jinendra. Saatam ni shubhkamnao. Aaje Uposhadh/Pratikraman thi aatma shuddhi taraf ek paglu vadhariye.
• Jai Jinendra. Saatam par aapne vrat, samyam ane pratikraman thi kshama ane shuddhata badhariye. Shubhechchha.
• जय जिनेन्द्र। सात्म के पावन अवसर पर उपवास/उपवासध और प्रतिक्रमण से आत्मशुद्धि बढ़े—शुभकामनाएँ।
• Jai Jinendra. On this holy Saatam, may your Uposhadh and Pratikraman deepen samyam and inner peace. Shubh Saatam.