Shop points and paypal

Hi , i want to buy some shop points to get premium stash so i've put 15$ in my paypal account but when i want to buy the point via paypal i am redirected to a page where i need to put my credit card number and stuff , the problem is that i d'ont have a credit card but i have 15$ in my paypal account so why can't i just pay with those 15$ already existing in my account ?

thank you in advance and have a nice day