code: func (c Client) Buy(amount float64, agreeBtcAmountVaries bool) (*transfer, error) {
  	params := map[string]interface{}{
  		"qty": amount,
  		"agree_btc_amount_varies": agreeBtcAmountVaries,
  	}
  	holder := transferHolder{}
  	if err := c.Post("buys", params, &holder); err != nil {
  		return nil, err
  	}
  	if err := checkApiErrors(holder.response, "Buy"); err != nil {
  		return nil, err
  	}
  	return &holder.Transfer, nil
  }
docs: // Buy an amount of BTC and bypass rate limits by setting agreeBtcAmountVaries to true