Currently, the cheapest method is Global Express Guaranteed at $40. It arrives in 2-3 days though, with tracking and all that. It's a service provided by FedEx through our postal system. Even though it is a total pain (mostly due to the amount of customs paperwork), we still do it because we want to be able to provide solutions for everyone.
We did offer a really cheap parcel post service but it was unreliable, took as long as 4 weeks to arrive (if ever), and had no tracking available. It didn't make for a very good customer experience at all. Since that doesn't fit our image as a fun and hassle free company with professional service, we dropped it. I'm still investigating alternatives.
Update: We have added International Priority mail service. It is a bit cheaper.