Is the Ecwid solution free?

Ecwid rates are available on their website:

WARNING: Our payment module cannot be installed with the free version.

If Ecwid is installed via a Wix website, you will also need to consider the following rates:

https://www.wix.com/upgrade/website

Please note that in this case it is not necessary to take the e-commerce offer, since cart management is handled by Ecwid.