I know there are too many different names involved in using Mixin Safe, but MixPay is a third party app on Mixin Messenger. They issues EPC for you so that you can verify your Mixin Messenger PIN while doing something. So you need to transfer EPC to them to get back the refund. In that procedure you proved that you knew the PIN, otherwise if they just sent back your TRX and you didn't know the PIN at all, then the money is lost.

And these small amount of TRX can't be transferred out of of Mixin Messenger because it doesn't even cover the withdrawal fee. So it's recommended to use it inside Mixin Messenger or MixPay.

MixPay is supported online in many places, like https://www.coinsbee.com/en/