Reason is that you paid with your work card (i.e. Business credit card or Corporate card). The cost of accepting such cards is much higher for LH than personal cards (the EU currently regulates the cost of acceptance for personal cards). LH is permitted to surcharge for Business/Corporate/Purchasing cards etc (aka Commercial cards) and of course non-EU cards. What they are not allowed to do is surcharge for personal cards issued by your EU bank. Try the same booking again and input your personal card details (without confirming the payment obviously) and you will see that you will not be surcharged