Do not repeatedly fold and unfold documents. Even 'invisible' tape contains a high acid content that will destroy the paper over the years. Better to live with the damage, or have the item professionally repaired. Do not laminate. Lamination adds acid and is usually irreversible.
Alternatives exist that are much better for protecting letters and documents.
They may be able to offer you some additional advice, tell you where you can find safe storage supplies, or at least refer you to a professional conservator. Leave repairs of these items to a trained conservator. Permission is given to all institutions serving the public to reproduce and freely distribute this pamphlet, so long as no changes are made to the text.