Hello Pangiea,
You have received the first Weekly Postmaster Sterling Treasure. The first Postmaster package unlocks the Chroma Grimoire Card, which is currently unlocked on your account.
I have just checked the postmaster and he/she has not given me my sterling treasure on any of my characters and I was wondering if I did something wrong and might have not unlocked it.
Hello,
You had already received your postmaster package since the last weekly reset. You will receive an additional package via the postmaster after each future weekly reset.