Uptime tools tell you whether your home page answers. They usually don't tell you that the "Pay now" button points to a deleted Stripe link, or that your Calendly page was renamed. The site looks "up" while customers can't pay or book.
If you only need to know that your home page answers, free uptime tools such as UptimeRobot or Notifier check it every few minutes. They don't follow your payment links or report broken pages inside your site.
Every day, from the outside: your home page and up to 100 linked pages (404 and 500 errors, broken redirects), the payment and booking links on your home page (Stripe, PayPal, Calendly, Planity, Payhip and others), and your HTTPS certificate, with an alert 21 days before it expires. You get an email the same day when something changes, in plain French and English, and a one-page report on the 1st of each month.
What it does not do: checks run once a day, not minute by minute. No access to your server or admin area, no backups, and it is not an antivirus.
VAT included. One website, everything included. No commitment: cancel at any time.
Monitor my website — EUR 29/monthUsually not. Most uptime monitors only check that a page answers with a 200 code. A payment link can be dead on Stripe's or PayPal's side while your page looks fine. You need a check that opens the payment link itself.
At least once a day for an online shop or a booking site, and right after any change to your prices, your products or your website.