Donald Sundman owns Mystic Stamp Company in Camden N.Y., which is a mail-order postage stamp dealer, one of the largest in operation and notable for both its promotion of stamp collecting as a hobby and for its acquisition of the one-cent Z Grill, the rarest United States stamp.

At the Siegel auction of the Robert Zoellner collection in 1998, Mystic president Donald Sundman acquired one of two known specimens of the Franklin Z Grill for $935,000, at the time the highest price ever paid for a single US stamp. Subsequently Mystic featured it in company advertising and listed it for sale in the catalogs, until it was traded for the unique Inverted Jenny plate block.