If you're dealing with websites with "one-click" purchases THERE IS NO ESCAPE from SMS! Impulse buying will triumph every time - esp. since you don't have to worry about where to put the books in your house any more, and can hide them from any concerned family members! SMS may be the new "dependency"!
On a related note, this ease of impulse buying seems to be totally unknown to the publishers who are delaying ebook release dates. How many of us have been browsing for one book, seen a new release and clicked that button?