If you have a UID number, you should click on "Business Purchase" after the checkout - then you can enter the UID number as well as the correct address and receive the invoice without VAT.
But this can only be done during the initial checkout process.
You may need to put your Country Code before the VAT number for it to be recognized.
As below, ensure you check out - do not use Paypal Express Checkout for business purchases.
For US Customers
Ensure you choose "Yes" for Tax Exempt?
For the Rest of World -
Ensure you change to Business purchase - at the top of the page.
Then enter your VAT number. Please cover over the "i" for help on how to enter.