Why can’t I buy a song on iTunes even though I have the exact amount of money?

I’m trying to buy a song that’s $1.29 and recently I bought a $20 gift card but now I have $1.30 left so I decided to use it on iTunes. I figured I’d use it on a song that’s between $0.99 and $1.29. I finally decided on which song and the song costs $1.29 so I should be able to buy it with one cent left over. After I do all the signing in, in order to buy the song, it tells me I have insufficient funds to buy the song even though I have just enough to buy it. I don’t know what to do or why it’s doing that but anything helps. Thank you!