The only additional charges are for use of the Phone & SMS features via the native Uptics Twilio Integration. If you don't use these services there are no monthly charges.
As listed on the pricing page, other subscription fees may include the addition of extra users (on top of the 2 users included this costs $79/user/month) and the addition of extra allowed inboxes or connected email accounts (on top of what is included in your plan this costs $19/inbox/month).