- set required fields https://wwws.echo-inc.com/ISPGuide-Fields2.asp
- counter field shouldn't be just a random integer (but it does need to
  fallback to that as B:OP doesn't require it)

- echecks / ACH
    #echeck use customer_org and account_type to generate ec_account_type

- check HTTPS response code and die if not 200?
- order_type & ccexp_month,ccexp_year setting fiddling ->{_content} directly
- 'echo_reference', 'security_result', 'mac', and 'decline_code' fields...?

- handle returning correct is_success and error_message values in the
  special case for AVS-only transactions we don't handle yet (B:OP needs a
  name for that transaction!)
- Hosted order type?