How do I add just a payment of 5 dollars onto iTunes?
I have a debit card and I want to upload 5 dollars onto my iTunes account for this book I want to purchase on iBooks.
If the card is valid and passes the iTunes validation you can add the card to your iTunes account, after which you can remove it : Change or remove your Apple ID payment information - Apple Support
Otherwise you might be able to buy an iTunes gift card (if they are available in your country ; though probably not a $5 card)