いつもアメリカに帰るとき通関するときに書く書類に

別送のことも書いてあったと思うので、ここ

http://www.cbp.gov/travel/us-citizens/sample-declaration-form

を見ると

15.If you are a U.S. resident, print the total value of all goods (including commercial merchandise) you or any family members traveling with you have purchased or acquired abroad (including gifts for someone else, but not items mailed to the United States) and are bringing into the United States.

とあるだけでした。

アメリカに住んでいるのでない人たちは　＄１００　から　＄１０００　が　グレイエリアで、　＄１０００を超えると 関税　が　つくようですね。

でも　こっち(インターネットでアメリカ外から物を買った場合）とかをみると

http://www.cbp.gov/trade/basic-import-export/internet-purchases

International Postal Service: Merchandise shipped through the international postal service is forwarded upon its arrival in the United States to one of U.S. Customs and Border Protection International Mail Branches for clearance. If the item is less than $2,000 in value and is not subject to a quota or is not a restricted or prohibited item, a CBP official will usually prepare the paperwork for importing it, assess the proper duty, and release it for delivery. This procedure is generally referred to as a mail entry. Packages whose declared value is under $200 ($100 if being sent as a gift to someone other than the purchaser) will generally be cleared without any additional paperwork prepared by CBP. However, CBP always reserves the right to require a formal entry for any importation and generally exercises this option if there is something unusual about the importation, or if important documents such as an invoice or bill of sale do not accompany the item.

とあるから　受取人が　アメリカの住人(年の半分以上をこちらに住んでいる人だと思う。　国籍に関係なく）
だった場合は　＄２０００　のばあい　関税がつくようです(郵便局ではらってくれってかいてある）

If any duty is owed, CBP will charge a processing fee for clearing your package. Duty and the processing fee are usually paid at your local post office, where your package is forwarded.