Even if you're paying in traditional currency (FIAT), you must choose the two checkboxes for data protection and withdrawal rights. You will be transferred to Stripe's payment service provider after clicking the buy button.

Stripe accepts three distinct payment methods: credit card, SEPA, and giropay. 

You confirm your purchase by clicking buy after filling out all of the required fields. 

Pay attention to the following: When the payment is processed, the purchase is complete. You will then receive a payment confirmation. 

After you've completed your purchase, go to your profile's Portfolio and Gallery section to check out your new Digital Original.
It is not yet an NFT because you paid it with FIAT initially.
Only by creating a wallet and minting it on the blockchain can you make an NFT of your purchase.