Why would anyone do that?
Stamps and postmarks are a big part of sending a postcard. Of the experience and treasure of a postcard.
(And of their ultimate financial value as collectibles actually.)
If you go to the trouble and expense of buying (or printing) a postcard, and then handwriting on it;
why would you then chuck a bunch of these in an envelope?
You're saving a few cents (maybe dollars) on stamps, but you've already spend dollars on the postcards anyway.
And you've already spent time and effort;
finding somewhere to buy them,
buying them,
(or taking a photo,
and printing it or getting it printed),
collecting your thoughts,
finding the addresses you need,
writing on them...
It's not much more time or effort to stick a stamp on and chuck it in the next mailbox you pass by.
I am a regular postcard sender and receiver and I don't think I would ever do the envelope thing you suggested as an explination.
Even if I can't always mail the postcards in the exact places they are from for reasons of practicality or lack of postboxes, I still get them in the post with a stamp somewhere.
Often I go to the trouble of going to a postoffice and getting them postmarked by hand to make sure the machines don't miss them, and/or to get a special postmark.
So.. your observation of the lack of stamps and postmarks makes me suspicious.
Did Gabby have a bunch saved up because they hadn't seen a mailbox for a while?
Did Brian forget to post them before travelling home? Did he just hand them to his sister when he got back?
Did he forget until he was halfway home, and then envelope them because if they all had the same postmark, that would look weird?
Or because he wanted to create the illusion that Gabby had mailed them from wherever she was supposed to be after she was dead?
Was he worried that if he posted them all individually, he'd be creating a trail of evidence of his movements via the postmarks?
OK maybe I'm overthinking it, but I still think it's a weird thing to do.