As for me Arthur I just created an account when I got my Kindle. Registered my Kindle and then registerd my debit card, all payments for stuff I get then go onto my Debit card.
Once it's registed, I don't even have to re-enter the card details, just log into my account and buy with one click. The books I get (whether free or purchased) are automatically download to my Kindle within a minute or two.
Just thousands and thousands of free (or very cheap) books available.
All done electronically at the push of a button