Auth.net Hosted Accept: User authentication failed due to invalid authentication values - authorize.net

Question

I created an Accept hosted integartion. This integration works fine in sandbox mode, but fails in LIVE mode with the following error:
User authentication failed due to invalid authentication values.
I verified that my transactionKey and api_login_id are correct and that I am using the correct URL.
The settings on the auth.net account are set to LIVE.
What else could be the issue? I see banter about requiring a signature key also, but nothing in the docs.
Below is the code for the request:
<getHostedPaymentPageRequest xmlns="AnetApi/xml/v1/schema/AnetApiSchema.xsd">
<merchantAuthentication>
<name>{hidden api_login_id}</name>
<transactionKey>{hidden transaction key}</transactionKey>
</merchantAuthentication>
<transactionRequest>
<transactionType>authCaptureTransaction</transactionType>
<amount>7.94</amount>
<order>
<invoiceNumber>138437174</invoiceNumber>
</order>
<billTo>
<firstName>First</firstName>
<lastName>Last</lastName>
<company>Company</company>
<address>Address</address>
<city>City</city>
<state>ST</state>
<zip>11111</zip>
<country>US</country>
</billTo>
<shipTo>
<firstName>First</firstName>
<lastName>Last</lastName>
<company>Company</company>
<address>Address</address>
<city>City</city>
<state>ST</state>
<zip>11111</zip>
<country>US</country>
</shipTo>
</transactionRequest>
<hostedPaymentSettings>
<setting>
<settingName>hostedPaymentBillingAddressOptions</settingName>
<settingValue>{"show": true, "required":true}</settingValue>
</setting>
<setting>
<settingName>hostedPaymentButtonOptions</settingName>
<settingValue>{"text": "Pay"}</settingValue>
</setting>
<setting>
<settingName>hostedPaymentReturnOptions</settingName>
<settingValue>{"showReceipt" : false, "url":"https://www.hiddendomain.com/plugins/authnet_iframe_communicator.html","urlText":"Continue","cancelUrl":"https://www.hiddendomain.com/plugins/authnet_response.php?action=cancel","cancelUrlText":"Cancel"}</settingValue>
</setting>
<setting>
<settingName>hostedPaymentShippingAddressOptions</settingName>
<settingValue>{"show": false, "required":false}</settingValue>
</setting>
<setting>
<settingName>hostedPaymentStyleOptions</settingName>
<settingValue>{"bgColor":"#B00"}</settingValue>
</setting>
<setting>
<settingName>hostedPaymentIFrameCommunicatorUrl</settingName>
<settingValue>{"url": "https://www.hiddendomain.com/plugins/authnet_iframe_communicator.html"}</settingValue>
</setting>
</hostedPaymentSettings>
</getHostedPaymentPageRequest>