Remember that something that sounds too good to be true, usually is.
There have been instances where pre-paid cards and vouchers have been sold by criminals engaged in black mailing and other criminal activities, and once you touch that card or voucher, your ip is logged, and if the case is big enough - you will have police at your door, and they confiscate equipment without asking you. This has happened. So stay safe, stay away. And if you chose to interact, do it with an ip that's not your own - and hide your tracks.
After some research it might seem like bestxchanger is legit, but be careful. Who gives money away online for free?
250 NOK what could that be, something like 42 usd, i think the risk that whoever uses the voucher will be on anyones top list of priority is very small. I have no ide how ucash works but i guess someone can just buy something digital behind a proxychain if there the least worried that there might be a small chance something is wrong with the vouchers origin.
Maby you have seen free fauctes giveaways, free coin giveaways for new altcoins, free money/btc to gamble on casinos, frerolls in poker(there used to be 10 000 USD freerolls in the past on the ipoker network for example) and so on, there is many people that gives away money/btc online, usually in small amounts thou.