This is just a heads up, the offer is NOT available until Friday. There is currently a $12 sale on this week anyway but if you wait until Friday all T's will be $9 each but just for 24hours.
Here's the blurb from their email:
All tees will be $9 for 24 hours only (Starts 11/27 12:01 am CT, ends 11/27 11:59 pm CT)*! So make sure you've got all your holiday gift choices all lined up and ready to go, fire up the Internet early in the mornin', and let the deals begin!
I think you still need to keep your order below £18 (roughly $30 but please check exchange rates before ordering) to avoid customs charges. The £18 limit is the value of your order before shipping costs, the shipping is not taken into consideration for import tax.
Also, I believe you can add stuff to your cart now and when you log back in once the $9 sale starts on Friday the items will still be there but at the lower sale price. This might be worth doing as often with Threadless sales the common sizes go quickly.