Yes. Every Pro feature is unlocked for 7 days. A card is required to start — we save it, charge you nothing on day one, and make the single $19.99 payment on day 7 unless you cancel first. That one payment is a lifetime unlock, not a subscription: there is no renewal after it.
Cancel anytime before day 7 from your account page on the web, or from Settings → Account in the Mac app, and you are never charged. You keep Pro until the trial's original end date either way — cancelling early does not cut the trial short.
If you would rather skip the trial, you can buy Pro outright at any time. Either way the 14-day refund window applies from the date of the charge.
The free tier (core blocker, unlimited blocks) stays free forever with no time limit.
Pro is account-keyed, so a single purchase covers every device you sign in on.