Imho, envelopes are a thing of the past. They used to be popular, BUT, paper often contains sulfur, which reacts with metal and can turn your coins yellow.
Nowadays, most collectors use 2x2s for most coins, and capsules for bu/proof coins when you want to preserve luster and prevent oxidation. For the purposes of storage, a slab is effectively an overpriced capsule. For coins hundreds of years old, they might be fine just sitting in a tray, since they've already developed an oxidized layer that protects the metal underneath.
Typically, you want to select a storage method that would best preserve the coin while costing no more than 10% of the value of the coin.