I stopped using this plugin for a few years and used the RCON method instead, because often times players get disconnected when making a purchase and the RCON method sends the command when the player is not online which forces me to go the plugin route. 

I find myself having to reload the plugin most of the time to get it to run the commands from the store. I already used the tebex contact page, but I'm pretty sure I sent them the same complaint a few years ago and the same issue still exists.

Yes - it has happened many times to me as well.  Don't expect any changes because this plugin hasnt been optimized in over 2 years.

I switched from the plugin to RCON a few months ago and have been miserable since due to the offline issues you mention. 
Going back to the plugin as soon as possible.

We need to spam every new social media post of theirs with this major issue.