You're out of USDX, what do you do now? Easy, just head to the Assets page (from the left sidebar) and deposit with your favorite cryptocurrency.
You can choose the cryptocurrency you want to top up with from Bitcoin, Ethereum, Dogecoin, USDT, or UTK. For USDT, you can either go for ERC20 or TRC20, which significantly reduces any transaction fees.
After the transaction is confirmed your USDX will be added to your balance.