[einhverfr]

I think this may be an important point, but it makes me wonder if "click here to order" would be less effective than "order today." Certainly the latter contains a call to action and perhaps a better one. Again it's a question but one which I have not studied in terms of user behavior.

[Evbn]

The former is a simpler more immediate action that is easier to induce.

[einhverfr]

The second is more imperative.

As I say, I don't have data on this, but it would be interesting to study.

[hrktb]

IMHO introducing a time dimension might be contrary to the goal. 'today' means later in the day perhaps (==never, acutally). You could use 'now', but it would still induce a reflection on doing it now or not. Just 'order here' feels like less to parse and less ways it could go wrong.