Panel Sampling allows external users to query your panel and find custom fields and their answers which they are interested in.
Incase one of them is interested in using your panel to send surveys, they may do so. Sampling Surcharge is the cost charged for each respondent who satisfies selected answer option(s).
We have a Sampling Portal where this is possible.
Yes. Currently, a field can have sampling disabled while edit/add field. A checkbox is provided for Disable Sampling.