I could sound dumb but what the fuck is an RBTC and how does it have the same value as BTC?  Is it not like any other Altcoin but just forked off bitcoin's network?

Anyway, you seem to have become a victim of their unsupported feature. Technically, they're not processing withdrawals of that coin so I'd assume that fee is probably auto-calculated. You should rather ask when they plan to support that coin and if they're not supporting it anytime soon you should ask them to refund your deposits.