This branch of Habitat for Humanity has chosen to charge volunteers for the privilege of helping out.
When the Nonprofiteer pointed out that volunteers give more readily to the agencies they serve than non-volunteers, she wasn’t advocating admission fees. Volunteers may have paid to paint Tom Sawyer’s fence, but Twain’s point was that they were stupid. Your volunteers aren’t.
Even if mandatory “contributions” (oxymoron watch!) weren’t offensive in suggesting that volunteers’ time has less than no value, they’re practically the definition of penny-wise and pound-foolish: people will pay what you require (or not) and then regard their giving to the agency as being done for the year.
Or forever. Please stop this idea before it kills again.