My advice to have the DH handle it and to just give her the stuff (or whatever DH doesn't want) has nothing to do with thinking the ex is reasonable, attached to the items, or entitled to them in any way. It has everything to do with making a relationship that cannot be severed because of the child as smooth as possible. I think the OP's involvement as opposed to the DH handling it would be a poor choice and I think choosing peace is best here.
This has given me a bit of insight. You are suggesting peace now and hoping it will mean peace in the future. I am suggesting to not give in now because I don't think peace now will mean peace later; I think peace now means more demands later, things that will be harder to give to keep the peace, which will result in the OP and her DH being doormats.
I agree. It sounds like OP and her DH already do what they can to facilitate a good relationship with the ex for the child's sake. I don't think handing over whatever household items she decides to demand is a good idea.
In this case I can see how the ex may have a claim to these items(in my opinion a very tiny claim). And she asked 3 specific items and 2 of them she mentioned were gifts from her mother. So I do think to keep the relationship good in this case we have to at the very minimum give her the gift items to not start a war.
There have been no's to requests in the past where she has demanded things that weren't hers to claim(requests to come stay at the house, requesting pickup/dropoff times she knew would not work for us). This situation does suck though because it does make us feel that all that we do will never be enough for her.
And you're probably right, whatever you do likely won't ever be enough for her. The thing is, you can see to it that there is a balance so that you feel you are doing something but not being taken advantage of. I think giving her the couple of things you are comfortable with is a good compromise. Try not to think of it as "We have to give her what she wants to avoid a war." but rather "We do most of the giving in this give-and-take but DSS is worth it." And DO continue to set boundaries because it won't do your rel
ationship any good to start feeling resentful, either.

I can tell you this - I grew up in a situation very much like your DS's and I am eternally grateful to my parents (Dad and Step-Mom) that they were careful to allow me to have whatever kind of rel
ationship with my Mom that I could. Sometimes it ended up being none but I always knew that her due to her choices and not theirs. After I was an adult my step-mom confessed to me that at times it had been extremely difficult to be cordial to my mom but she was glad she and my dad had been determined because my brother and I were worth it. That said, my mom never tried to come back and stake any claim on household goods. I don't imagine that would have gone over well at all. Kudos to you and your DH for trying!!