Could you provide us supporting evidence for your claim?
TBH, I don't see megapari completely ruining their reputation for 20 bucks, so you'd probably get it back assuming there are no other underlying issues.
And that was few years ago, they are a different casino now, maybe @W617854 you sent it on a wrong channel or have follow up on the wrong support. Since you are already in the forum, if you still got that evidence of your claim, then you are always free to create a scam accusation even if the transaction happened years back. I'm sure that $20 can easily be paid once you get their attention, but like @Potato Chips have mentioned, a proof is necessary.