Every little bit helps

"There are two ways for you to make a contribution to reduce the debt:

• At Pay.gov, you can contribute online by credit card, debit card, PayPal, checking account, or savings account.
• You can write a check payable to the Bureau of the Fiscal Service, and, in the memo section, notate that it's a gift to reduce the debt held by the public. Mail your check to:
  
  Attn Dept G
  Bureau of the Fiscal Service
  P. O. Box 2188
  Parkersburg, WV 26106-2188"

https://www.treasurydirect.gov/govt/resources/faq/faq_publicdebt.htm#DebtFinance