I just tried your ideea ... set to x_cust_id "aaa" . Checkout the params that came back (I set the relay response just to show those):
Params : {"x_test_request"=>"true", "x_amount"=>"10.00", "x_ship_to_state"=>"", "x_ship_to_first_name"=>"", "x_cust_id"=>"", "x_city"=>"", "x_auth_code"=>"000000", "commit"=>"Purchase", "x_ship_to_city"=>"", "x_ship_to_last_name"=>"", "x_email"=>"", "x_state"=>"", "x_avs_code"=>"P", "x_ccv2_resp_code"=>"aaa", "x_MD5_Hash"=>"7436E52B8056625B7FD0DD52DD4C28F7", "x_fax"=>"", "x_company"=>"", "x_account_number"=>"", "x_first_name"=>"", "authenticity_token"=>"O0PykS09KbfR6qLwnCdPWpvFtR0+EQwi6Ul70sZiuqI=", "utf8"=>"\023", "x_po_num"=>"", "x_invoice_num"=>"", "x_country"=>"", "x_address"=>"", "x_last_name"=>"", "action"=>"relay_response", "x_ship_to_country"=>"", "x_ship_to_zip"=>"", "x_ship_to_address"=>"", "x_ship_to_company"=>"", "x_type"=>"auth_capture", "x_trans_id"=>"0", "x_tax_exempt"=>"FALSE", "x_tax"=>"0.00", "x_zip"=>"", "x_card_type"=>"", "x_response_code"=>"3", "x_duty"=>"0.00", "x_freight"=>"0.00", "controller"=>"authorize_net", "x_cavv_response"=>"", "x_phone"=>"", "x_cvv2_resp_code"=>"", "x_description"=>"", "x_method"=>"", "x_response_reason_text"=>"(TESTMODE) The referrer, relay response or receipt link URL is invalid.", "x_response_reason_code"=>"14"}
Also I made a typo mistake and wrote x_ccv2_resp_code and that seems to be recorded. Can I create any custom fields that I want to , cause if that would be the case than there would be no problem for me...
I would just have custom_id instead of x_cust_id and everything would be fine.