I agree that I would personally understand why @giuseppe96 would refuse doing so, but I still don’t think even delivering flowers to a gay wedding is in any way cooperation with evil. At most it would be remote cooperation with evil and could be justified based on the fact that flowers are not necessary material for the commission of the act (mediate cooperation) and the contribution thereof does not lead to the commission of the act (remote cooperation). I take the same position on cake baking, picture taking, etc. Catholics do not need to feel compelled to refuse such services in fear of cooperating with evil. However I also support personal discretion in this and believe we need conscience protection laws in place for individuals who might come to a different conclusion than I do.
Welcome to CAF.
I wouldn’t be bothered myself by delivering flowers to a gay wedding, but I can fully understand why another Catholic might be troubled by it and wish to opt out.
We are supposed to pray for the dead. Do that for the deceased, the family and all attending this funeral while you are on this delivery. It is the Christian thing to do.