code: func (c *Client) GetBilling(i *GetBillingInput) (*Billing, error) {
  	if i.Year == 0 {
  		return nil, ErrMissingYear
  	}
  
  	if i.Month == 0 {
  		return nil, ErrMissingMonth
  	}
  
  	path := fmt.Sprintf("/billing/year/%d/month/%02d", i.Year, i.Month)
  	resp, err := c.Get(path, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	var b *Billing
  	if err := decodeJSON(&b, resp.Body); err != nil {
  		return nil, err
  	}
  	return b, nil
  }
docs: // GetBilling returns the billing information for the current account.