You used to be able to, once you selected what type you wanted (say, 10gpt) and it appeared on your/their offer, you could select that and it would come up with a dialog box where you could fine tune it. It stopped happening a few weeks ago, I'm hoping they'll fix it soon.
By the way, when you do dip favour, it becomes invisible. That might trip you up when it appears they're offering you free stuff, they're not...