This is a completed form CN 22. It is supposed to be attached to the outside of international envelopes containing anything other than letters. This one was attached to an envelope which contained an Indian token I won for ₹220 (about US$3.46) in an eBay.in auction.
I've noticed that coins sent from India always have such a declaration. I recently received a 2013 coin from Poland of similar value which did not have a declaration. About half of the low-value coins I receive from Europe seem to lack it.
According to the US Customs web site, a customs duty must be paid for all merchandise worth above $200. (Gold coins have an exception and there is no duty on them but they still need the forms). According to cpb.gov, you must pick up goods worth more than $2,500 in person. I have never imported coins at that level.
Sellers generally don't want to attach the declarations to packages with no duty, as the declarations themselves are believed to encourage theft. If there is no Customs Duty to be paid or high-value reporting requirement it seems a pointless theft risk to affix a declaration. US Customs x-rays the envelops and will open them if it looks like duty might be needed. Yet the web site warns that the forms must be present accurate.
I was surprised to learn from cbp.gov that the weight of the item is expected. The form I show here does include it, but this is unusual in my limited experience. An this case the weight was not accurate: the token I received was about 1/3 the weight listed.