From Europe, you can mail one package per day to yourself in the US, worth up to $200 duty-free (mark it personal purchases). If you mail an item home valued at $250, you pay duty on the full $250, not $50.
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Mailing and Shipping Goods - Customs Duty GuidanceUp to $1,600 in goods will be duty-free under your personal exemption if the merchandise is from an IP. Up to $800 in goods will be duty-free if it is from a CBI or Andean country. Any additional amount, up to $1,000, in goods will be dutiable at a flat rate (3%).
For goods over the £1,500 threshold and restricted and excise goods, and goods over 1,000kg you will need to make a full customs declaration either by yourself or through an agent.
You must declare all items you purchased and are carrying with you upon return to the United States, including gifts for other people as well as items you bought for yourself. This includes duty-free items purchased in foreign countries, as well as any merchandise you intend to sell or use in your business.