My partner sent me and our mutual partner this message;

"There is no hurry or rush for this, but I would like to have some help with getting Kelly [fake name] a gift for christmas."

I don't know how to respond to this kindly, letting them know that they need to manage this on their own. Me and mutual partner have already gotten Kelly a few little gifts for this year, and I don't want to help my partner with this since it's not my relationship, and is unrelated to me. But when they have asked for help finding a gift for one of their close friends or family members in the past, I have done shopping for them and just put both of our names on the gifts.

It's totally a me issue that I will do this kind of thing for them for other people, but not for Kelly. I can see my own unfairness, but it brings up a lot angry, jealous, and disgusted feelings to think about helping them get Kelly a gift.

I basically need advice on how to turn down this "request" for help, because I can't come up with a sensitive enough way to say that I don't want to help them. Or I need advice on how to help, but not just do it for them, since I don't want to just run their relationship with Kelly for them.