Does an email thank you letter for an online donation differ from a thank you letter that you mail to a donor?
No. It still needs the fundamental characteristics of any other thank you letter. But it may include some additonal information or capabilities, such as links to more information. Most organizations that receive online donations do say thank you with an email acknowledgment immediately. But don't just dash off an email thank you. There are a number of best practices now for email thank yous, and you should follow them.
It is also wise to follow up with a mailed thank you, especially if the donor is new to your organization. Retention is very poor for new donors acquired online, so anything you can do to encourage an online donor to bond with your cause may help retention and increase your fundraising results.
Here are some examples of emailed thank you letters from a variety of charities.