Every initial Google Workspace activation is charged immediately upon purchase (as with any other product activation). Subsequent renewals are rolled up on the renewal date and charged as a single purchase.
However, if an additional seat is added at a later date, the cost of the additional seat will be charged immediately and upon renewal every month as a separate charge.
For example, if a Partner activated Google Workspace | Business Starter and 1 extra seat on the 1st of January, the wholesale price is charged immediately and will renew on the 1st of subsequent months. If the same Partner adds one more seat for the same account on the 10th of January, the cost of that one seat is charged immediately and will renew on the 10th of subsequent months as a separate charge. (Meaning there will be 2 separate charges within a month - one for the initial activation and the other for the additional seat)