Per the Department of State Website https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html
"A letter of invitation or Affidavit of Support is not needed to apply for a visitor visa. If you choose to bring a letter of invitation or Affidavit of Support to your interview, please remember it is not one of the factors used in determining whether to issue or deny the visa."
ISSS no longer issues these letters and encourages applicants to follow the guidelines on the above webpage.