Yes, Let's Encrypt certificates are free, you don't need to pay nor deactivate it ;). The mail you are receiving is a mail informing that one of the issued certs will expire in 30 days so you can take action to renew it.
I don't use Plesk nor its Let's Encryp plugin but I suppose that this plugin should renew the certificates automatically, maybe the expiration mail you received is for one certificate that you issued but you don't use... if you provide your domain name we could check it.
If the list of hostnames of a certificate has changed, Let's Encrypt will see this as a "new" certificate and will notice you for the expiry of the "old" certificate.
So if something has changed in the contents of your certificate and you are positive the current certificate for your site(s) isn't closeto expiry, you can just ignore the e-mails.
Your certificate (or certificates) for the names listed below will expire in
19 days (on 13 May 17 11:44 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
Yes, you only have issued one cert for your domain https://crt.sh/?q=%boksclub.com and it will expire 13th May. Indeed, your site is not using this cert, it is using an invalid cert provided by Plesk.
As I don’t use Plesk I can’t give you any procedure but yes, you need to check it on your Plesk Panel because right now it is not configured properly.