The small gift of a link
For every customer in Keep Up there's a private web page. You send them the link once (email, text, WhatsApp, print it on the invoice). They tap it. They see a friendly schedule with their next visits, last visits, any messages, and a pay button.
Add to Apple, Google or Outlook
On their page is one button: Add to calendar. It's an iCal subscription — meaning as you change the round, it updates in their calendar. No work for you, no surprises for them.
In their language, with RTL
If the customer's language is Arabic, Hebrew or Farsi the page flows right-to-left with the right greetings. Spanish, Polish, Portuguese — warm, polite, human phrasing. Not Google-translated.