Just do what I did:
Once when I was about nine years old I wanted to buy a bottle of rum for my mum for mothers day.
I went to the first counter with an old fat woman about 40 years old and she said "blah blah blah no alcohol unless you're 18!"
Then I went to the second counter with a young blonde chick (looked about aged 17-20) and she asked why I was buying alcohol but when I said it was for mothers day she let me buy it
Except that time I actually did get it for my mum but you can keep the Diablo 3 thing for yourself.
Just buy it like a week before mothers day.