Authorize.net: cannot pass auto-incremented invoice or description in custom amount payments

I am passing a number of tickets and ticket price that is auto-calculated for payment using the Authorize.net widget. In other forms of payment, using the "sell products" option, we have descriptions and the UniqueID widget passing a unique invoice number. However, I cannot choose the UniqueID in the Authorize.net widget when I'm using custom amounts. We cannot use the "sell products" option for this, since we need to gather guest names based on conditions and I can't pass the number of tickets to the sales widget.

Is the authorize.net widget buggy for custom/calculated amounts and invoices, and if not, is there a way we can pass a description and invoice to authorize at all? I'm concerned that a variable in that option isn't being defined right or perhaps quotation marks that are missing or need to be omitted .

I have also attached a screenshot of the timeout error that we are seeing regularly with our use of the form, after long loading times.

As I can understand, you're trying to pass a custom field in your transaction, this can be possible by opening the payment wizard and clicking on the option "Additional Gateway Settings", there you will find the option to set up the custom field: 

Regarding to the error you're currently receiving, I've cloned the form a integrated it with Authorize.net on my end, I was able to successfully submit the form, here is a screenshot of it: 

Please try re-integrating your form with Authorize.net and if the problem persists, please let  us know. 

For some reason, this will only let me choose the "Payment ID Copy", which I have used Conditional Formatting to copy over. It does not let me choose "Payment ID" as I would like it too. I have a redundant hidden field - also the problem shows in the authorize record on your end - it's not showing the Payment ID. Your screenshot shows this.

In another form I have, Courthouse Badges, Authorize showsPayment ID from Unique ID widget, it's not even specified as the invoice number in the widget at all but still transferring.

The difference between this form and and the courthouse badges is that courthouse badges was made in March and uses "sell products". Retirement Tickets is a couple weeks old, and uses "donation/custom amount" options. I have plans for another large form that would need to use this form of payment and will report the UniqueID properly to Authorize.

In the Retirement Tickets, the Payment ID shows up, but it does not show up in Authorize once I've added letters to the uniqueID.

Courthouse Badge invoice number in Authorize.net:

Confirmation e-mail from Courthouse badge:

The Unique ID widget does not seem to be recognized as custom invoice number in the Authorize.net integration, the value provided by this widget cannot be used in calculations nor conditions either, this is why you cannot see the custom invoice number in my payment submission shown above, because I selected Payment ID Copy and it did not have a value, in your form you're currently copying the value from the Unique ID widget the the second field, but conditions do not recognize the value of Unique ID widget either.

What I would suggest you is to use a different widget, the Random Value Generator will help you to also create custom invoices numbers and even get the value with conditions, then you may copy its value and paste it in the Payment ID Copy field, here is a screenshot of the condition that you may set up in your form: 

Also, do note that in the "Invoice Number Field" you can use Text Boxes fields only, any other field type is supported to be passed as custom invoice number.