So seems like forever now that as you play, which I think is Bungie's concept, "play the game". Yet while we do, the potential to lose out on Shards, Prizes, Ascendant Alloys are always present.
We get NO WARNING of full postmaster until its too late and already going to Tower. I've farmed for DAYS to prep for upcoming season and just had this happen, this system is totally and seriously stupid that we can't have a "pre warning" that we are almost out of space instead of it telling you once you've lost items. We don't even get a warning while in orbit, unless we proc going to Tower, then it pops up.
I'm no game designer but man, this is seriously upsetting to many in the "we play since D1 kinda crowd" and it MUST BE FIXED... gone on for waaaaay too long without a fix. Grrrrrr
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Bungie and the community here will tell you over and over that the postmaster is not to be used as a vault. But I agree that it should notify you when it gets almost full at any point in the game.