iTunes and iPod Touch "Payment was declined"

I recently bought a new iPod Touch 5 and made 3 purchases. The last app I purchased downloaded successfully onto my iPod but I never got an email receipt from the store. So I checked my purchase history and it says > "We were unable to authorize your credit card for this purchase. > Please update your billing info." So I went to the "Billing Info" page and validated my PayPal account. Now it says > "Your PayPal transaction was declined." I tried adding a credit card as my payment option and I get: > "Your credit card was declined. Please enter valid credit card information."
What should I do? Thanks