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.
- Books of bus tickets that are sold cheaper than buying individual rides: Most poskim consider this yesh lo, since the buyer may use the bus rides all up immediately after buying the book if he so chooses. (Another heter would be based on the fact that the reason the book of tickets is sold cheaper is, like the magazine subscription, to guarantee the bus rides being bought, or to make it easier for the drivers not to have to count change.)
- USPS forever stamps: Note first that ribbis does not apply at all to non-Jews, whether they be the borrower or the lender. But, in this case, even if the US government would be completely Jewish it would not pose a problem for the same reason as above. If the consumer wishes, he can use all the stamps he buys immediately after purchase. Thus, the fact that now that the price is higher, he is in essence benefitting from his advance payment is not a problem.