"Therefore, we are seeking corporate and small business sponsorships in the form of commercial advertising," the letter read.
Brown said the letter was sent "in error."
"After performing some due diligence, (reading Board of Ethics opinions, consulting with advisors) we determined that the proposed mailing should not include the WURD letter," Brown said in an e-mail Wednesday.
She said her staff prepared a mailing with the WURD solicitation, an office newsletter, and information on financial literacy - the kind of packet she regularly sends to constituents and supporters. Her staff was told to remove the WURD letter from the 290 mailings, but about 30 were included by mistake.
Brown's mailing included a "sponsorship packet" for businesses to advertise with the show. She was not paid for cohosting the program.
Blogger and Kensington neighborhood activist Christopher Sawyer obtained a copy and posted it on his website, Philadelinquency, on Tuesday.
Brown said she was unaware that some letters had been mailed until hearing from reporters.
WURD, which Brown touted as the only African-American-owned station in the state, has a talk-radio format that includes covering City Council's weekly meetings live from the chamber.
Brown said she did not seek an opinion from the city's Board of Ethics, but determined internally not to include the solicitation. She did not write a separate solicitation on her private letterhead.
Because she had no financial interest in the radio program, it's unclear whether the letter would have violated any ethics rules. (Board of Ethics executive director Shane Creamer said Wednesday he could not comment.)
The closest precedent in recent cases appears to have come when Councilman Dennis O'Brien asked whether he could solicit businesses to sponsor a race to benefit youth sports groups. The Ethics Board gave its approval.
Brown has had some clear-cut trouble recently.
Last year, she and her campaign committee agreed to pay a record $48,834 in fines and repayments for a series of omissions, misstatements, and misrepresentations on campaign finance reports.
Among the issues, she admitted using $3,300 in campaign funds to repay a personal loan from Chaka "Chip" Fattah Jr., son of U.S. Rep. Chaka Fattah.
Brown, however, blamed her 2011 campaign manager, John D. McDaniel, for attempting to cover up the payment.
McDaniel later was sentenced to a year and a day in federal prison for stealing more than $100,000 from her campaign.
Brown later backed off plans to hold a fund-raiser, in part to pay off the fines, after the Ethics Board expressed misgivings about a public official paying personal debts from "gifts of money."
Brown, an at-large Council member, is majority whip, making her the third-highest-ranking member of the body.
Some political insiders have speculated that her recent troubles could make her vulnerable at the polls.
But Brown, known for championing the issues of "women, children, and families," has proven popular with voters. She received the most votes among at-large candidates - more than 66,000 - in the 2011 Democratic primary.