Its a security thing that google does on mobile clients. They limit your purchases until you upgrade your account at https://checkout.google.com/upgrade
from the mobile device. I think this just makes you set a pin number so that it is harder for someone to steal the device and use your google account to buy stuff.
I should probably detect iOS devices and put up a warning. One workaround right now is to just add one race at a time to the cart and checkout, then go back and add the next one.