Due to the new U.S. customs regulations, sending gifts valued over $100 or commercial goods via regular postal mail is no longer possible until further notice. In such cases, both private and business senders can use DHL Express for shipping.
You can find official U.S. government information on the background of these changes here (Executive Order) and here (Fact Sheet).
Shipping to the USA with DHL/Deutsche Post will only be possible again once the necessary processes have been implemented. This is because the USPS, in cooperation with DHL, must collect customs fees from the recipient and forward them to US Customs.
DHL Express operates in the USA itself and is not dependent on the USPS, so it can collect and forward the fees itself.
Picture: U.S. Federal Government