Good instinct asking about doses first, because with preservatives the dose IS the safety. Honest map:
Calcium propionate is the first choice. 0.1-0.3% of the FLOUR weight, into the dry ingredients. Safe, works against mould, and it's the only one on your list that's fully bread-friendly at working doses. Fine in cookies and cakes too.
Potassium sorbate: 0.03-0.04% of total BATTER weight - note the different base, that's the trap that catches people. Very safe, good in cakes and fillings, usable in breads without meaningfully slowing the yeast.
Sorbic acid is a surface spray only. 1g per litre of water, misted on finished cooled products. Never into a yeasted dough, it kills yeast. The "liquid" in its ratio means the spray water, not your batter.
Sodium benzoate... last resort. 0.1% of batter if nothing else fits, but reach for the first two.
Three honest things. The gain is DAYS not months - a 5-6 day product stretches to 1-2 weeks, nothing makes a 4-day courier trivial for a soft product. Use max one or two per product. And these doses need a jeweller's micro-scale, 3g in a 3kg batch is not a kitchen-spoon measurement.
For courier specifically, the preservative is one leg of four: fully-dried bakes, heat-sealed packs with silica, and courier-hardy products (dry cakes, biscotti, cookies over anything soft) matter just as much. And "no preservatives, small batches" is itself a premium position if you'd rather keep the label clean.
