This is a limitation of Paypal Direct Payments, it will not work with UK Accounts.
For customers in the UK, you can Use Authorize.net as the payment gateway or use payPal Express.
For the USPS error:
API Authorization failure
you need to contact USPS to have them upgrade your account for use on the production rate lookup server. After signing up for an account, you need to contact them to have it verified on the production server before it can be used.