Bitcoin payments are presently only available for US-based customers. Since Coinbase is unable to convert other currencies to BTC, only US customers can pay with Bitcoin.
It's a pity. This means I will not give them my money ;-)
No more HumbleBundle till BTC payment will be available to me.
So basically, they won't let you pay in BTC because they charge more for the bundle if you're not paying in US$.
Two good reasons to go elsewhere to get your fix.