Magento – Paypal Express Checkout redirects to cart for United States

You can do the following if you don't want to change core files:

1. Create folders Mage/Paypal/Model/Api under /app/code/local/
2. Copy the file Standard.php from /app/code/core/Mage/Paypal/Model/Api
3. Paste the file Standard.php to /app/code/local/Mage/Paypal/Model/Api
4. Change the line 238 from

$request['address_override'] = 1;

to

$request['address_override'] = 0;

As explained in here also Magento's multishipping checkout does not support payment methods that required a redirect to a payment gateway.
The reason for that is beyond my understanding.
And making the multishipping support payment gateways is not an easy task.
If I would want to implement this I would start by looking at how the onepage checkout handles redirects to payment gateways and how it handles the feedback from them.