Posted: 04/10/2015 at 17:11
Had another look, and it's basically down to the 'order code' being correct, and then the 10 for 10 deal should apply automatically as long as all your choices qualify e.g they are single packs, not doubles or collections.
If you go from the Facebook link, your own email invitation, or twitter link, then the 'order code' box should be populated automatically with the appropriate code. If you go via a FB link, the code is the same as the one mentioned by Perki, TNE592Z.
If you can't / don't want to follow the above, then manually putting in the code should work for you. If you've previously browsed and not cleared the cookies, then an old order code may have defaulted, and again, putting in this code manually should work for you.
This is the link generated from the T&M FB page: