Now you have a bunch of prepared covers and/or postcards. Awesome! You need to keep them safe, and you might want to display your favorites. You have a few options.
You can put your covers or postcards inside plastic or glassine sleeves and store them in a box. Transparent sleeves got a justifiably bad reputation in philatelic circles for a long time because it turned out that many of them were not safe for long-term storage. Collectors turned to translucent glassine instead.
The problems with transparent sleeves have been (to the extent of experts' knowledge) eliminated now, and reputable stamp supply houses should carry safe sleeves. Do a search for "archival poly sleeves" on a stamp supply website and you should see the options available to you.
Here is where you are going to be glad you used #6-¾ envelopes and 4×6 postcards.
Stamp supply dealers carry specialized "cover albums" that are made to fit #6-¾ envelopes. They also carry "postcard albums", but if the size of the pockets is 4"×6", it will be more affordable (and you will have a much greater range of options) if you look instead for an acid-free photo album, as 4×6 is a standard photo size.
I like doing this, especially for gifts. The easiest way is to buy an off-the-shelf frame with glass, a matte, backing, and hardware for hanging or displaying on a table. These are priced reasonably at craft stores.
To mount your covers, my favorite way is to get a piece of acid-free craft or construction paper of the right size and attach a cover-sized philatelic mount. The size you are looking for, in pre-cut mounts, is 165mm×94mm.