I thought this was a bad idea.
And I'd like to go into why:
You're trying to build a relationship with your donors. It's like dating. You send them an appeal - asking them out. They give you a small donation - saying 'ok'.
Everything that happens between now and the next time you ask them for money is the date. How do you want it to go?
How you treat them will determine if they go out with you a second time.
Sure, sending an email will save you the costs of printing and postage. But what about the impression it makes - or doesn't make?
Will they read it?
If your donor doesn't read your thank you message, it's the same as if you never said thank you at all. And it's up to you to make sure they read it.
So send it to them in the way they are most likely to read: Printed, through the mail.
Donors (even the small ones) are the lifeblood of your organization. Make sure they know they are appreciated - this is not the place to cut corners.