Yes. You can choose the billing style that suits you from the Packages & Add-ons page, which has two options:
Custom plan (flexible, monthly)
A flexible monthly plan where you choose exactly how many Maildoso (SMTP) mailboxes, Google Workspace mailboxes, and domains you want, using the sliders.
Mailboxes are added in bundles (minimum 10 at a time), and the per-mailbox price depends on how many you have in total — larger volumes get a better per-mailbox rate.
On the Custom plan, domains are purchased separately at $12 per domain, per year (a fixed price, with no volume discount).
Packages (quarterly bundles)
Quarterly bundle plans that come at a set size, with domains included in the package.
A good fit if you want a ready-made bundle rather than configuring each item yourself.
Google Workspace mailboxes are billed monthly. You can view current sizes, prices, and add-ons directly on the Packages & Add-ons page. For how mailbox and domain charges appear on your bill, see Understanding your bill: mailbox plans vs. domain charges.