Congratulations on taking the initiative! There's nothing like personal communication with donors.
You can add/edit the text of your thank-you letter through the charity area. (click here for a guide how to log in as charity)
Here's a guide on accessing the charity area and editing your thank-you message:
1. Log in to the charity account
2. Clink on My account > Charity Name
3. Click on Edit charity page button
4. Locate the section for thank-you letters to customize your message in Hebrew and English