For every transaction on the ether blockchain, and thus for every purchase of UnitedCrowd Token, a transaction fee (Gas) has to be paid. When buying UnitedCrowd Token you have to determine the Gas Limit, which is the maximum amount of Ether you are willing to spend for your transaction to be performed.
If you provide too little gas the transaction will fail. Therefore, it is important to always specify a sufficient amount of gas. We recommend to use https://ethgasstation.info/ for appropriate estimation of the required gas.