Short answer: proration is time-based and depends on the remaining billing period + unit price of the plan.
When you add seats mid-cycle, the preview should:
- Calculate remaining time in the current billing period
- Multiply that fraction by the per-seat price
- Add that as a prorated charge in the upcoming invoice (or immediate charge depending on config)
Common reasons it looks “off”:
- Billing cycle anchor mismatch (e.g. reset vs fixed)
- Quantity changes applied with delay or at next cycle
- Taxes or discounts applied after proration
- Rounding at invoice level vs line-item level
Since this is calculation-specific, best to have the team inspect your exact case.
👉 Please share:
- Subscription ID
- Plan + price per seat
- Quantity before/after
- Timestamp of change
And reach out here:
If you want, paste your preview response here and I can help break it down.