To be honest...I think it's not etiquette to write that on the invite. I think it's perfectly fine to ask for money and contributions to honey moon just not on the invite. So if anything on the invite just ask them to check out your wedding website for registry information. Then on the wedding website indicate that gift cards are appreciated or contribution to your honey moon. Flight Centre offers a registry where people can contribute online or in person at a flight centre to your honeymoon as a gift. You should visit Flight Centre.
Click on each tool to see a sample.