I just rewatched the scene in question, and the envelope in the safe deposit box definitely has an object the size and shape of the toy airplane in it. Subsequent scenes make it clear that it was.
Why do you think we see the same 10-12 characters most of the time? Everybody else is off behind the bushes…
He was probably willing to keep it a secret not so much for worry over Kate getting the blame, but to protect Sun from being even more shunned by her husband.
I guess there’s a little bulge, but it still doesn’t seem big enough to be a toy plane.
I agree. I thought it was firmly established that the toy plane was in the safe-deposit box. That added to the mystery of why she would go to the trouble to robbing a bank just to get it. We were left with the questions of who it belonged to and why she wanted it. Was that the episode where she said " I killed the man I loved" ?
Yes, it was when Jack was asking her why the plane was so important.
Here are a few different shots so everybody can compare and make their own decision:
I can definitely see a bulge that could be the toy plane in that first photo, Monstre
Man, are we overanalyzing this thing or what??? 
Obsession can be fun.
Nah, we haven’t even started discussing what make & model of envelope & toy plane it was.
DC-3. National Envelope, #10 brown kraft envelope 
Oh, I agree. It’s Thor’s dad you’ve got to convince… 
Now if I were a gay man, I would come in and authoritatively say, “I know a bulge when I see one!”
But since I’m not gay, I hereby give permission for somebody thusly qualified to use that joke, as long as I get royalties…
Does it have to be a gay man? I can say it as a straight female, but it may not have the oomph you were hoping for.
“I know a pair of bulges when I see one!”
Yes, but what is the serial number of engine #2?
So, if Kate was to get on the raft and they got picked up, to what lengths would she have gone to protect her identity? Was she going to smash the radio so that the ship couldn’t radio ahead that they had picked up survivors of Oceanic flight 815? Was she planning on killing the other lostaways before being picked up so that they couldn’t say where they came from and what her real name is?
I’m thinking Kate is a professional assassin, a la Grosse Pointe Blank. She did a short stint in the military before being turned out by the CIA as a “wet” operator. Now she is a private contractor and that is why she has a vehicle set up with several different plates and she picks up her mail, payments, and assignments from hotel desk clerks. The letter aprising her of her mother’s condition was sent by her handler.
Perhaps if I just sent you my picture…
… said handler being really, really bad at his job.
I have to admit that I was thinking it would not be healthful for the other three guys if Kate was on the raft. Otherwise, we’re talking another Prancing Pony situation if they get rescued. “Kate? Oh, yeah, Kate, she’s right over there…”
R-1830-90-92-88-15
I think Kate is, by far, the most dangerous person on the island (aside from the “others” maybe). Jack should have listened to the marshall!! She’s the very definition of “femme fatale!”
Yeah, I’d still do her…
Yep, and she’s certainly the sneakiest. Of course, now we know she’s had years of practice, evading the law, going around in disguise and with various license plates.
And yeah, I’d still do her, too.