There are two ways you can save your payment method in Nettla:
When you make your first purchase, Nettla will suggest that you save your payment method for future purchases, You can choose to opt out of this option if you want.
Your profile -> Settings -> Payment methods allows you to add a card manually that is then saved for your future checkouts.
Note: If you pay using Apple Pay or Google Pay, there is no need to save your payment method, since those payment methods only require one click as opposed to adding your actual card information.