code: func (s *CertificatesService) PurchaseLetsencryptCertificate(accountID, domainIdentifier string, certificateAttributes LetsencryptCertificateAttributes) (*certificatePurchaseResponse, error) {
  	path := versioned(letsencryptCertificatePath(accountID, domainIdentifier, 0))
  	certificatePurchaseResponse := &certificatePurchaseResponse{}
  
  	resp, err := s.client.post(path, certificateAttributes, certificatePurchaseResponse)
  	if err != nil {
  		return nil, err
  	}
  
  	certificatePurchaseResponse.HttpResponse = resp
  	return certificatePurchaseResponse, nil
  }
docs: // PurchaseLetsencryptCertificate purchases a Let's Encrypt certificate.
  //
  // See https://developer.dnsimple.com/v2/certificates/#purchaseLetsencryptCertificate