The discount is valid for gift cards purchased online or in stores, yet be warned that the supplies are limited. Another reason for you to act quickly in case you want to take advantage of the discount is that the offer is available today only. In other words, Best Buy's $50 iTunes gift cards will cost $40 for only 8 more hours or so, as of this writing.
For anyone who is interested, the source link below will take you to Best Buy's page. Let us know down in the comments in case you got one!
source: Best Buy via 9 to 5 Mac