You can control what you get from faction rank up packages (Destiny)

by Kahzgul, Monday, June 27, 2016, 13:30 (3413 days ago) @ Chappy

I see the same thing - if the drop is for a gear slot that's already full it goes directly to the postmaster. If I rank up multiple ranks at once (thank you heavy ammo synths), the second or third package can't be redeemed until I leave the vendor menu and then return.

For my test it was only one rank up at a time, and when I filled my inventory except for the special weapon slot, I had to talk to him 7 time before I could finally accept the package.