We are sorry if you paid more for the plugin you referred to before it went into our current promotion. As you probably know from our emails, though, promotions drive plugin sales heavily.
We try our best to offer high-quality software at competitive pricing, but we hope you understand that we can’t refund older invoices whenever a plugin sells again. It’s pretty typical for plugin prices to go down over time in various promotions, but we are sure you have been able to enjoy and use the plugin successfully already during the time you have owned it.