You can create new WEBIDs (panel log-ins) and give those WEBIDs specific panel privileges, limiting what they can do in the panel. See the link below;
You can also place the domains under the control of separate users. When a particular user logs in via FTP, they will be placed in their own home directory, which contains the web accessible directories for the domains that they control, they will not have access to other user's domains.
Yes, promo-code discounts apply to setup fees. Since setup fees are only charged on monthly plans, you should be aware that the max promo-code discount allowable for monthly plans is somewhat lower than the overall max of $97, but the max allowable discount is still enough to cover the setup fee.
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