Orders completed with no shipping selection

[FatalFlora]

Hi all.

I've got a problem with my store where purchasers are able to complete a sale without selecting a shipping option, so I'm having to email them and then ask them to pay shipping costs before I'll send their order (it defaults to the free postage option that I have every time).

It appears from my testing that this happens when the express checkout is used, with the traditional checkout your forced to pick an option. Does anyone have a fix for this? 

Thanks