I'll add in to concur with John that in my experience, while a year end tax receipt might have some functional tax related benefit for the donor, the biggest value of sending that receipt is in the opportunity it gives you to thank the donor (even if you already have - several thanks per gift is recommended and productive). Your receipt creates another valuable moment where your organization is top of mind and making them feel good about their impact.
Further, while this is far down the list of reasons to send a mailed receipt, since you mention cost as a concern, don't forget to include a reply envelope. At many organizations, this results in a meaningful offset to the cost of the mail, and in some reports, it appears to contribute positively to donor retention.
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Mike Fischer
Sage70
mike@sage70.com------------------------------