First, we need to identify whether the free account you mentioned is actually MXPlan or Zimbra, as the mail‑account limits differ.
If it’s MXPlan, I understand you’ve activated the free hosting option with your domain. If so, you have two options:
Upgrade your free hosting to a paid solution that includes more accounts (Starter-2; Perso-10)
If you don’t want to move to paid hosting, you should delete all of your free hosting to “free” the MXPlan reference and then purchase a Standalone pack as you were trying to do, with 5, 25, 100 or 1000 accounts.