Payment by credit card:
Add a license to your shopping cart, click the Pay Now button, fill in the requested personal data, choose payment by credit card, click next. Now in Your order you can see a blank field between VAT (%) and total price - this is the place to fill in the VAT ID.
Payment by PayPal:
If you choose paying via Paypal (instead of a credit card), you will be redirected to your Paypal account. Log in and click Change in the Pay with line. Once you are satisfied with the payment method, click Agree and Continue. This will not process the payment, you will instead be taken back to FastSpring to confirm the payment. You will come to a screen where you can enter your VAT ID.