Witter, based in Orlando, Fl., started the website in 2004 after a casual conversation with his friends about what believers might want after they leave their non-believing loved ones behind. And he said he hasn't changed the website since it first launched.
Witter, an atheist, charges $4.99 to $799.99 to deliver a pre-written letter to those loved ones. Ritter said he suspects the postal service and email services will not be available anyways.
Witter, the postmaster general of the Post-Rapture Post, said he has only sold his simplest letter product at $1.99, although he does offer more elaborate options. For $800, a calligrapher, a friend of his, will hand write your letter on "medieval parchment style paper."
Witter, who has another day job, said there has not been renewed interest because of the May 21 campaign.
He said there were about 200 letters when the website first launched, but now it "trickles in." In general, Witter said he receives more business from his merchandise, including mugs and t-shirts, with a ratio of 10 to one, than his letter products.
"I'm lucky if I get one every couple of months," said Witter about letter sales, who has sold about 400 letters in total. "This new event really hasn't added much."
He said 95 percent of the messages are from Judgment Day believers who want to encourage their loved ones that it's not "too late" to believe. The other 5 percent are jokes.
Kevin Thompson, co-owner of Northwest Shelter Systems, based in Idaho, said concerns about a nuclear disaster -- not Judgment Day -- have driven recent sales of his hidden rooms and bomb shelters.
"We're not a doom-and-gloom company by any means," he said. "People are still purchasing shelters from us for a number of other reasons."
Sales have increased 60 to 70 percent since the start of this year, he said. He attributes the growth mostly to the tsunami and earthquake in Japan in March, and especially the resulting concerns about radiation emitting from the Fukushima plant, north of Tokyo, Japan.
Thompson estimated that his company has built more than 300 shelters in 21 years of operation.
Thompson said his company is also a certified contractor for the government. Northwest Shelter systems have cost from $50,000 to $20 million, although, Thompson said, the average is about $1.4 million.
Barbie Grossman, who is media director for the Vivos Group, which is a company based in Southern California that builds underground shelter networks, she said there has been a diverse range of sales inquiries but the company has not yet built a network for a group expecting Judgment Day.
Vivos builds the underground networks for groups of 112 to 1,000 people that in some ways are luxurious hide-out accommodations that include food, water and power. Members from one large group in central Europe paid $25,000 per person for continuous ownership.
"It's yours forever to pass onto your children or grandchildren," she said. "If nothing happens, it keeps on going."
Jerry Jenkins, the writer behind the "Left Behind" series of books about the apocalypse, said he has a growing number of media requests regarding the May 21 campaign, although he is not aware of a respective increase in sales.
Jenkins worked with the pastor, Tim LaHaye, for the series' 16 books, which have sold more than 63 million copies, the first published in 1995. Jenkins said the books have been re-released this year with new covers and updated words related to technology in the series. He said the re-release was planned last year and related to the series' 15th anniversary, not the May 21 campaign.
Jenkins said it is "folly" to try to predict the second coming. "I think we're supposed to be ready, wait and watch. But I do think God is merciful and wants more to believe and not be left behind," Jenkins said.
Gunther von Harringa, spokesman for Family Radio and president of Bible Ministries International, said it is "ludicrous" to think that businesses will be able to function after Judgment Day.
"There's going to be a worldwide earthquake when many millions are going to die, and all the cities around the world are going to come crashing down," he said. "The idea that someone is going to profit from this is absurd."
He also said animals are going to die just as humans will.
"It's going to be a struggle just to survive for humans," he said. "The world as we know it is going to be radically altered and everyone will be in the same boat."