Every day you are offered to purchase special packs called bundles or special offers. Every few hours, you get a different bundle. Under the offer icon on the main game screen, the timer displays when this offer expires.
The pool of bundles is the same for every player. They are created manually, their contents and prices are fixed, and their distribution is a fully automatic process. There is no limit to how many times you can be offered the same bundle. If you miss one, you will eventually see it again.
In addition, you can purchase a bundle only once. Therefore, you can buy every single bundle in the game. However, new offers are added to the game regularly.