What do you need to include in your donation acknowledgment letter? Knowing how to write an in-kind donation letter for your nonprofit means understanding the do's and don'ts.Let's check them out! My research on thank you letters culminated in a list of the 20 Characteristics of Great Thank You Letters which was published in DonorCentered Fundraising. A gift acknowledgment letter weaves together your nonprofit's appreciation, engagement, and compliance efforts following a donor's generosity. A donor acknowledgement letter should include a few specific items to assist the donor and help your organization comply with IRS regulations. Acknowledgement letters should be mailed no later than one week (57 business days) from the date of the gift. There is no official IRS form that the exempt organization has to complete. When does the nonprofit need to send the written acknowledgment letter to donors? Example: "Thank you for including our organization in your estate plan with a legacy donation.