code: func (h *Handler) GetProductIncome(
  	tkn, productID string) (*uint, error) {
  	logger := h.logger.Add("method", "GetProductIncome",
  		"productID", productID)
  
  	return h.uintFromQuery(logger, tkn,
  		`SELECT SUM(channels.receipt_balance)
  		     FROM offerings
  			  JOIN channels
  			  ON channels.offering=offerings.id
  		     	     AND offerings.product=$1`, productID)
  }
docs: // GetProductIncome returns total receipt balance from all channels of all
  // offerings with given product id.