Are there any additional steps to be able to reference a custom payment method in a web service call? I have been creating sales orders successfully via SOAP using the method of "Cash" like so:

<ns01:paymentOption internalId="1"/>

. A new payment method was created but changing the call to send

<ns01:paymentOption internalId="31"/>

results in this error:

Invalid paymentoption reference key 31 for entity xxxxxx.

Sales orders can be created successfully in the UI using the payment method for the same customer, subsidiary, item, form, etc. Why would the method be unavailable in the web service context?

usual causes are people not knowing the difference between a payment option and a payment method

I have double checked the internal id so that's good. I won't claim any expertise so may be caught in the payment option vs method though it's not for a lack of research. Payment method returns an error when I try to set it but that may be a different configuration issue.

how are you getting the internal id of the payment option

The payment method field (id paymentoption) contains the options from the payment method list so that's where I'm looking. I get that there is a difference but the terminology doesn't seem consistent across schema and documentation.

the only relation between payment methods and payment options is that when you enable payment instruments

That's more descriptive than anything I've seen so far. Separate records, separate ids. That's what I would expect. I'm pulling the dropdown values using getSelectValue and seeing the difference. I'm thinking the right fields might not be displayed on the forms. Is the payment option list visible in the UI somewhere for users to reference?

you shouldnt see a payment method field on transactions after you enable payment instrucments

It seems the payment option field is labeled "Payment Method" which lends to the confusion. Maybe that's specific to our account.