Ether
Here you can top up Twei on your User Account as well as withdraw Ether. Since Twei is in fact a denomination of Ether you don't need to worry about exchange rates. A Twei is always one millionth of one Ether (1 Twei = 0.000001 ETH).
However, Ether itself is highly volatile. So, it's worth keeping an eye on the exchange rate of ETH to USD. This is a reference one retrieved from CoinGecko and Chainlink.
ETH/USD
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Yesterday
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MetaMask
If you want to top up Twei on your User Account or withdraw Ether you need to have MetaMask installed in your browser(*).
(*) Not all web browsers are currently supported by MetaMask.