Thanks for using Let’s Encrypt for your customers! That’s terrific.
I hope you have taken a look at
According to the second one, you can accept the Let’s Encrypt terms of service on behalf of your users if you are the one who keeps control of the private key for the certificate. This is true for many hosting providers.
You don’t need any special agreement with Let’s Encrypt in order to provide certificates to your customers. The regular terms of service of the Let’s Encrypt service are available to individuals as well as to hosting providers, whether small or large, and it’s fine to use Let’s Encrypt services as part of a paid or for-profit enterprise.
For the trademark, my understanding is that that policy does not grant permission to use the logo; maybe @josh could explain ISRG’s views about the appropriate use of Let’s Encrypt trademarks by hosting providers who offer Let’s Encrypt certificates.