findByProdID(prodID, column)

/**
 * Finds the requested value stored in the given column in
 * the Products sheet, for the given Product ID.
 * @param {string|number} prodID - The Product ID to look up.
 * @param {number} column - The column number to look in.
 * @returns {string} - The value in the given column for
 *   the given product.
 */
function findByProdID(prodID, column){
  // Find the given Product ID in the Products sheet.
  var filteredValues = allProducts.filter(function (o){return o[prodIDColumn] === prodID });
  // Return the value from the given column for that Product ID.
  return filteredValues.map(function (o){return o[column] });
  // @TODO: error handling
}