I'm in the process of building a game in which virtual items can be obtained either by in-game play (defeating enemies, gaining levels), or by purchasing "packs" via microtransactions.

Looking at an existing example like Duels.com's item packs, it looks like a lot of thought went into their implementation, including:

• Setting clear player expectations as to what can be obtained in the pack
• Limiting pack supply to increase demand and control inflation

Are there other considerations that should be taken into account? For example, should the contents of the packs be pre-generated to guarantee the advertised drop rates, or is each drop rate just a random chance, and you could end up with higher or lower supply?