Direct access to someone's inbox is a privilege. They have invited you to communicate with them, and that is very powerful. The CAN-SPAM Act outlines how not to abuse this.
If you are looking for a thank you gift that has meaning, one option is giving charitable gift cards. Instead of buying stuff, it allows the recipient to donate the card amount to a nonprofit of their choice.
Do something good with your 404 page on your website. Amplify your favorite non-profit causes.