In reply to New Max:
I've no experience of this but I suspect you might run into difficulties, since climbing gear isn't listed as one of the items which can be transported as sports equipment. There seems a fair chance they will check, since a duffel bag won't look as if it contains permitted items.
https://wizzair.com/en-gb/information-and-services/travel-information/bagga...
The idea of the sports equipment allowance is to carry large and/or heavy items which otherwise wouldn't meet the criteria for hold luggage - skis, bikes, golf clubs etc. Climbing gear can be carried as normal luggage, and it seems to me there's a considerable risk you'll end up having to pay full whack for a piece of conventional hold luggage and won't get a refund for your 'sports equipment':. Read the Q&As: If you check in fewer items of baggage than you had paid for, the baggage fee will not be refunded.
Check with the airline, and if they say it's OK get that in writing.