I sell jewellery and other smaller items, and rarely need to send them as a parcel. I generally use small padded envelopes, but rigid card mailers are good if the item is a little more fragile, or would be suceptible to bunching up in transit (eg long, thick chains) and making the envelope more than 2cm thick.

Both the padded envelopes and rigid mailers are available on eBay at pretty good prices if purchased in bulk.