code: func (pcf *Pay) PrePayID(p *Params) (prePayID string, err error) {
  	order, err := pcf.PrePayOrder(p)
  	if err != nil {
  		return
  	}
  	if order.PrePayID == "" {
  		err = errors.New("empty prepayid")
  	}
  	prePayID = order.PrePayID
  	return
  }
docs: // PrePayID will request wechat merchant api and request for a pre payment order id