Note the Core Rulebook is only a good use of this code, if you're into hard copies. If you like the PDF, get the humblebundle instead (includes core book and others at even just 1 dollar!).
If you like PDF files, I recommend paying just above average (currently about 18 bucks) to unlock a bunch of things, unless you both live in the US (and thus the shipping isn't stupidly expensive) and want something physical in which case make it 25 bucks (note: budget an additional 10 to 20 for shipping). Of course pay more if you both feel like it and can afford to.
Then continue the shopping spring on here getting what ever wasn't included with the discount...
However, if you are buying something physical anyways (and thus pay shipping cost) then how about adding this for 2(!?!?!) bucks:
If I didn't have to pay so much shipping (to Canada), I'd totally buy that last one... with or without 10 % discount :D