VAT refund for an online order from Germany shipped to Spain?

I am in Spain but I am not a resident of the European Union. I made an online order to a German seller who shipped the product to Spain. I got an invoice that includes Spanish sales taxes (21%).

I asked the German seller to provide me with a DIVA form so I can apply for a VAT refund. They told me they can't refund the sales taxes in such a way that it would be in conformity with German tax law.

I believe that Spanish sellers always have to provide a VAT refund (via DIVA form or invoice stamped at the airport). Is this optional for German sellers, even if they charge sales taxes of another country?