Every generation is billed in credits. New accounts start with 1,000 credits free, and credits start at $1 per 100 — so there is no plan to pick before you have seen what the agent produces.
Three monthly plans, each a credit grant rather than a feature tier — the product is the same on all of them:
Yes. One-off packs never expire into a commitment and need no plan:
New accounts get 1,000 credits at signup and no card is required to spend them. There is no recurring free allowance after that — further generations need credits, bought either as a pack or as part of a monthly plan.