Is there annual billing?
SentryDock is billed monthly only: Starter is $30 per month, Pro is $100 per month, and Ultra is $300 per month. There is no annual plan, so there is no yearly prepay to weigh up — you pay month to month and can cancel anytime.
If you subscribed on an older annual arrangement, it stays as it is; the billing portal and your invoice remain the source of truth for your renewal.
Switching plans
Change plan from billing settings or the customer portal the checkout uses. Timing of renewals follows the invoice you already have — read that invoice rather than assuming a custom proration.