Hello,
Okay, so your configuration will add the group "Seller Premium" to the customer and that group will be removed when the subscription expired.
The goal of the plugin is to "add" a group to the customer ; Joomla 3 do have a multiple-group feature.
The Acymailing subscription plugin have more settings, because it can add a group when the subscription start but also when the subscription expire.
Maybe, more options for the group subscription could be interesting, to also have something to "remove" group when the subscription start ; and another to add group when the subscription expire (but it won't be possible to know what has been removed in order to restore it).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.