Assalaamu alaikum.
The following hadeeth might help:
Chapter: Regarding gratitude for acts of kindness
باب فِي شُكْرِ الْمَعْرُوفِ
Narrated Abu Hurayrah:
The Prophet (ﷺ) said: He who does not thank the people is not thankful to Allah.
Arabic text of hadeeth:
حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا الرَّبِيعُ بْنُ مُسْلِمٍ، عَنْ مُحَمَّدِ بْنِ زِيَادٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ يَشْكُرُ اللَّهَ مَنْ لاَ يَشْكُرُ النَّاسَ " .
Grade : Sahih (Al-Albani) صحيح (الألباني) حكم :
Reference : Sunan Abi Dawud 4811
In-book reference : Book 43, Hadith 39
English translation : Book 42, Hadith 4793
Link:
http://www.sunnah.com/abudawud/43/39
You could also, when you write thanks, write a small dua for them, such as "May Allah reward you, ameen. "