A Buffer account is required to use this plugin. Buffer has paid tiers as well offering more scheduling options. This plugin is meant as a connection to buffer only and the customer should understand that scheduling and configuration is done in their buffer account, not on the WordPress side.
While Buffer handles the social media scheduling, the scheduled posts inside WordPress still run on wp-cron. This is only triggered upon login to the site, so often the scheduled posts do not get posted at the desired time. The web developers are actively looking for a resolution, but please do not promise your customer that scheduling posts within the WordPress account will work flawlessly.