Hi dotonpaper,
I just wrote to the mail address you gave me, but meanwhile I just have the opportunity to make some tests and apparently I found out that your latest plugin version (2.1.5) has this problem with the WordPress version 4.4.1.
You can check this because on my english website version (https://www.rnd60.com/booking), where i have the WordPress version 4.4.1 and your latest plugin version (2.1.5), your plugin doesn’t calculate the total amount of the booking and it doesn’t allow you to select the book button.
Instead on my italian website version (https://www.rnd60.com/it/prenotare/) I restored the previous backup where I have the latest WordPress version (4.4.1) but i kept your plugin to the version 2.1.3 and apparently here it’s working perfectly as before.
Following this it seems to be more an incompatibility problem between your latest plugin version and the wordpress core rather than a problem of my website.
Please let me know what to do being that your plugin is really nice and well done but without these functions working correctly it is useless and I need to get it fixed asap because it’s actually impacting my business too.
Thank you