Usually this is because the specific credit card is not setup for international payments. Contact your bank to authorize payments to Europe, which is typically a very easy fix.
It is based on the shipping address, if within Europe, the VAT (Btw or MwSt) is automatically calculated and visibile on the invoice. Outside of Europe there will be no VAT charged. Customs is likely to charge custom clearence fees which vary per country.
SSD ships worldwide, exceptions being Russia, North Korea and Iran.
It depends. SSD does it utmost to keep the website updated but it varies per product. Products such as bipods and other accessories are typically in stock. Optics typically take some 1-2 weeks to reach our warehouse. Custom firearms and ammunition can take roughly 1-4 months.
The Netherlands, Germany and California (USA)