It's been a problem for a lot of people for a long time now, blues should not have priority over ascendant shards and prisms in the postmaster or anything really. I can't think of any material or item that I wouldn't mind being deleted because a random blue just got sent to my postmaster and has taken its place. I always have 10 shards and 50 prisms in my postmaster for each character and I have to be super careful with how much stuff I let get sent to my postmaster, I know people also have other exotics and other materials in their postmaster cos they have no space anywhere else. I don't need blues, I do need crafting materials though. Blues as a whole drop way too often from activities and they're just a massive nuisance overall.
Please just add an option to auto dismantle blues or at least delete the blues that go into my postmaster if they're about to take the spot of an ascendant shard or other high quality material and remove it.
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So if you always have that much they are not needed. Post master should be cleared if you want it. I finally got some sets and load outs I have put together. When I maxed out my materials on my characters and stuff is in the vault I take action and use materials. Use it or lose it. Sorry if you have too much of something and it is a burden.