Complete a conditional acceptance and include the law below:
18 U.S. Code § 8 - Obligation or other security of the United States defined - The term “obligation or other security of the United States” includes all bonds, certificates of indebtedness, national bank currency, Federal Reserve notes, Federal Reserve bank notes, coupons, United States notes, Treasury notes, gold certificates, silver certificates, fractional notes, certificates of deposit, bills, checks, or drafts for money, drawn by or upon authorized officers of the United States, stamps and other representatives of value, of whatever denomination, issued under any Act of Congress, and canceled United States stamps. (June 25, 1948, ch. 645, 62 Stat. 685.)
Also let them know they may be committing mail fraud by continuing to send you statements (bonds) without any updates to account after they have accepted your coupon.
18 U.S. Code § 1341 - Frauds and swindles - Whoever, having devised or intending to devise any scheme or artifice to defraud, or for obtaining money or property by means of false or fraudulent pretenses, representations, or promises, or to sell, dispose of, loan, exchange, alter, give away, distribute, supply, or furnish or procure for unlawful use any counterfeit or spurious coin, obligation, security, or other article, or anything represented to be or intimated or held out to be such counterfeit or spurious article, for the purpose of executing such scheme or artifice or attempting so to do, places in any post office or authorized depository for mail matter, any matter or thing whatever to be sent or delivered by the Postal Service, or deposits or causes to be deposited any matter or thing whatever to be sent or delivered by any private or commercial interstate carrier, or takes or receives therefrom, any such matter or thing, or knowingly causes to be delivered by mail or such carrier according to the direction thereon, or at the place at which it is directed to be delivered by the person to whom it is addressed, any such matter or thing, shall be fined under this title or imprisoned not more than 20 years, or both. If the violation occurs in relation to, or involving any benefit authorized, transported, transmitted, transferred, disbursed, or paid in connection with, a presidentially declared major disaster or emergency (as those terms are defined in section 102 of the Robert T. Stafford Disaster Relief and Emergency Assistance Act (42 U.S.C. 5122)), or affects a financial institution, such person shall be fined not more than $1,000,000 or imprisoned not more than 30 years, or both. (June 25, 1948, ch. 645, 62 Stat. 763; May 24, 1949, ch. 139, § 34, 63 Stat. 94; Pub. L. 91–375, § (6)(j)(11), Aug. 12, 1970, 84 Stat. 778; Pub. L. 101–73, title IX, § 961(i), Aug. 9, 1989, 103 Stat. 500; Pub. L. 101–647, title XXV, § 2504(h), Nov. 29, 1990, 104 Stat. 4861; Pub. L. 103–322, title XXV, § 250006, title XXXIII, § 330016(1)(H), Sept. 13, 1994, 108 Stat. 2087, 2147; Pub. L. 107–204, title IX, § 903(a), July 30, 2002, 116 Stat. 805; Pub. L. 110–179, § 4, Jan. 7, 2008, 121 Stat. 2557.)
Hope this helps you on your journey.