Would like to see a code sample: Create SOP Sales Invoice in one function, apply a SOP Payment another function

I'm pulling my hair out over here trying to get this to work.  Using C# I can create SOP Sales Invoices fine.  When I attempt to add a payment, I either create a new

taCreateSopPaymentInsertRecord_ItemsTaCreateSopPaymentInsertRecord  with (from what I can tell) absolutely no association to the invoice I am attempting to apply a payment |or| I receive an error from eConnect complaining about using a duplicate document number input.

Surely there is working code sample out there somewhere that demonstrates how to do this

I can post my code here, but I really do not think that would help.  I would much rather see something that works.

Frustrated and ready to punt.

Thanks to anyone who can give me a nudge in the right direction.

Don