It's a great idea to put it on the invitation. This covers the etiquette of if people receive an invitation, in etiquette standards means the meal is paid for. If this is stated on the invitation then all the boxes are ticked. At the end of the day who pays is not important, I doubt any one will even bat an eyelid and they will be thrilled to be there to celebrate a remarkable feat 45 years!

Enjoy the awesome party!