Occasionally Apple offers a special bonus if you would like to pre-fund your Apple ID account. From now until March 14, Apple is offering a 10 percent bonus for up to $200. So if you add $200 to your account, you get $20 extra added. Your funds are then used for iTunes purchases, App Store purchases, and recurring charges like iCloud storage and Apple Music.
To get this deal on your Mac, in iTunes go to Account, View My Account. Then look for Add Funds to Apple ID on the right under Apple ID Account. On iOS, click on your Apple ID in the Settings app, then iTunes & App Store, then your Apple ID at the top, then View Apple ID, then Add Funds to Apple ID.