Does USPS get antsy if you dont write a name or a legit name on the package?
No, but I've had them give me weird ass looks when I try to file an insurance claim on a package with weird names on them.
I know they've definitely given me a hard time if I don't know the recipient's name.
But usually I send the package with the person's real name but in the return address section, I'll put my address only. Sometimes I'll write VGC Secret Santa so they know what it might be. If the post office demands your name, folks here will probably only know you by your username anyway
I put my username inside the gift package because I'd rather they find out who the Secret Santa is when they finally open it, whether that be when they receive it or on whatever day they decide to open it.
I personally like to wait until Christmas morning to find out what's in the gift and who sent it to me. In my wishlist, I'll probably ask the person to put their username inside the box so their identity doesn't get spoiled