The management of the golf course in Verona, Wisconsin, said the idea was to remember the victims of the September 11, 2001, attacks.
TumbleDown Trails Golf Course also received death threats, CNN reported.
The golf course had placed a small black-and-white advertisement on Monday offering to "commemorate" the 12th anniversary of the terror attacks with "9 holes with cart for only $9.11 per person or 18 holes with cart for only $19.11!"
"It's an abomination and a disgrace," one person wrote.
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"To make an opportunity out of a national tragedy is outrageous," another said.
The management of the course posted an apology on Facebook yesterday and on a recorded message on the course's main phone line.
"Please accept our sincere apologies," the recording said.
"Our attempt to remember the tragedy of that day has been met with negativity and now even death threats. We ask that you please take a moment to reflect and remember those we lost that day, as we were trying to do."
"They shouldn't have to apologise for anything! Who are you people to judge," one supporter wrote.
Meanwhile, the management said in a Facebook post the course may be closed today because of security concerns.