While we haven't used it in the scenario you're discussing, my shelter has had success using a web-based scheduling software called Acuity Scheduling. It's inexpensive (there's even a free trial), accessible over the internet and various stakeholders can create individual appointment calendars under one account. It automates email confirmations and reminders, and you can design the emails that get sent, so I could see it including your voucher/certificate. It can also text, and text or email reminders to not only the customer but staff as well. It also allows you to accept payments. You might look into it. https://acuityscheduling.com/ and here is an example of what an embedded schedule link can look like
https://egasschedualing.as.me/vaccpass
Good luck!
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