This answer to another ribbis question indicates that deals of the form "subscribe and save X% over news-stand prices" are ribbis and thus forbidden. Does this principle apply to other items you pay in advance for, such as (1) books of bus tickets that cost less than the sum of the individual fares and (2) USPS "forever" stamps that you can buy before prices go up? In both cases you are paying less than the future face value, but I've never heard any complaints about either of these cases. Are they problematic too, and if not, what's the difference?
There is a principle when it comes to this form of Rabbinic ribbis called yesh lo. The idea is that if the seller (who is compared to the borrower) has the item in stock, and theoretically he can pass ownership of it to the buyer immediately, then it does not violate this Rabbinic enactment, even if time elapses and the price of the product increases.
The cases you listed would not violate this form of ribbis for this reason.