The very safest way to package unmounted artwork is in wrapped in archival paper in an oversized tube mailer with foam rolled up in the centre and stiff card around the outside. For mounted, I would recommend using an oversized flat cardboard mailer with the print sandwiched within an archival bag and between two layers of hardboard. Regarding shippers, there tends to be either cheap to moderately expensive but not very safe or very expensive but safe with nothing in between. I'm not sure where you are but most parcel firms and the Royal Mail here in the UK pay no attention to fragile or do not bend labels. That is the reason I suggest oversize mailers as the edges tend to get damaged most and hardboard helps protect from punctures in the centre. The safest way to ship if the prints are high value is to use one of the specialist couriers for artworks and antiques though the service prices tend to be quite high.