prayerletters.com can take the burden of printing and mailing receipts and year end statements off your plate. They specialize in printing and mailing newsletters for missionaries, but their experience applies equally well for organizational mailings.
To get started, make sure you're running DonorWise 4.1 or newer. Then, in the "System Setup" (under the "Tools" menu), in the "Printing/Mailing Service" section, click on "Connect to a Printing/Mailing Service".
On the "View Contacts" screen, click "Group Actions" and then "Send Year End Receipts to Current Group":