No need to buy protective pouches. If you have access to a laminator you will just need two laminating pouches to make your own, custom sized pouch,
Cut one pouch, ie both sheets, to the size of pouch you want. In my case this was a tad bigger than my folded letter (which was weirdly folded when posted out).
Separate the trimmed sheets and flip them so the inside faces out.
Place the trimmed sheets neatly inside the other pouch. Position them centrally as you will trace excess plastic off later. (With smaller projects you will get skilled at reducing wastage).
Laminate the pouch as you would usually. When it emerges, trim around the inner pocket leaving a narrow margin. Cut one short end off to access the inside of your pouch.
I used a hole punch to create a tab but this isn’t necessary. Some people re-laminate the finished pouched to seal the cut edges but I don’t find this necessary.