Just a note about the URL, it is listed in a business area, but the information for individual returns links to the same location.Current IRS guidance is at the link below. They could have made it very simple and clear if a 5% non-refundable deposit was the only way to have a "written binding contract".
The Plug-In Electric Drive Vehicle Credit (IRC 30D) - Internal Revenue Code section 30D provides a credit for qualified vehicles, including passenger and light trucks.www.irs.gov
As is always the case, there are vagueries written into everything so legality can be challenged, but the basis for the "binding contract" is not defined by federal law. As cited in the URL above, the binding contract is defined by state law, and the 5% everyone is so focused on is given merely as an example.
...a written contract is binding if it is enforceable under State law and does not limit damages to a specified amount.