Paid version is built differently

This plugin does what it says on the tin. My one big problem was that, after paying for the premium version, I discovered that all of my CSS testing was useless. The free version uses tables to build the calendar and external CSS (i.e. from Jetpack) works fine on it. The paid version, however, is built modern with DIVs and loads its CSS stylesheet at the bottom of the web page, including a few !important commands, making customization tricky, unless you load your own CSS from an even farther-down customized footer, like in a child theme. My conversation with support was very cordial (one email per day), but in the end, went nowhere. They say they need to brute force the CSS like that in order to avoid conflicts. They recommended that I edit their plugin’s CSS, but that would get blown out if my client ever updates. I recommended that a future version should include the ability to add custom CSS in Simple Booking Calendar’s admin panel, if this is how they need to roll. So, if the free version is enough for you, go for it. If you need a feature in the paid version, and you want to do any styling, make sure you know how to deal with the differences above.