The voucher itself doesn't cost fees when you buy it, but some merchants may apply a fee when you pay with it. While most casinos carry the deposit fees for their players nowadays, some might not. Take a look at the casino's fee policy to find out if they charge fees for deposits and withdrawals with Neosurf vouchers.
If you don't use your voucher for longer than 6 months, the company may apply a fee of €2 per month (or currency equivalent). It will be deducted from the voucher's value.