Hi, Andreas

Thanks for showing interest in Amelia

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We will try to answer all your questions as best as we can. If i miss any details, please correct me, and i will make sure to cover everything.

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The time unit is 15 minutes, minimum booking time should 

be 90 minutes.

- I am not sure what you mean by minimum booking time, but if you'd like, you can set initial time of 90 minutes , and add an "Extra section", where customers can choose to extend the time by 15 minutes, in increments.

Extras allow you the possibility to add additional services or products customers can buy when they are scheduling the appointment. By click on the “Add Extra” button, new options will appear:

Here is an example of a test page, how it looks for the customer booking on the front-end.

You can read more details about setting this up here.

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The idea is that the calendar should automatically add 15 minutes

service time for cleaning.

Sample:

customer1  books from 10:00 until 11:45

customer2 can book from 12:00

system blocks the time from 11:45 until 12:00 for cleaning.

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Yes, you can add a "buffer time" after the appointment, for this service; and you can set 15 minutes;

then it will be exactly as you described. 
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Additionally, I just wanted to point out that we have pages with Demos, that you can use to try out some of the plugin's features - https://wpamelia.com/demos/

and there is a 15-day money-back guarantee period,
so if you purchase the plugin, you can safely fully test it out,

we would assist you with our support along the way,
and if it doesn't fit your needs you can request and receive a refund in that period.

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Let me know how that sounds, and if you have any questions. Thanks.