You've been through an awful lot in the last seven weeks.
I think your plan is good - they can have things if they send postage. The items can be sent surface or book rate or something like that. It is slower but cheaper.
And if it is something you want to keep, keep it and hope that they're too far away to get the idea to come and get it. If they could come get it you may want to move it off your property and have someone else hold it for you.
It's fairly obvious that they want things they can use or sell. They may not realize that the WWII stuff is valuable. You may want to move that stuff to a safety deposit box if you want to keep it, or sell it right now if you don't want to keep it.
As far as I know, they have no legal title to anything he owned, you can verify that with a lawyer(consultations are usually free or around 25 dollars). Unless he left it specifically to them in a will, I don't think they have any rights to it.
I've seen in our family, that some people get very, very selfish and unpleasant when someone dies.
It's a very, very weird thing to see in motion, and it's also very painful and unpleasant.