Bundles are themed pairs made up of a Hammer and a Gem that can be obtained by buying them with Credits or Robux, or can be found together through crates or trading. Bundles only appear when there is a special event going on, for example christmas, halloween, etc. All bundles cost either 650 robux or 1500 in-game credits. The items in these bundles are the ones that most of the time have the most valued and get traded the most in the Trading Post.
When do they release?
They release on (special) holidays. For example there will be christmas bundles for christmas, halloween bundles for halloween, etc.
They release on game milestones. For example when it's the anniversary for the game they always will add a new bundle.