This is most certainly NOT solved.
The ability to draft invoices with multiple payments is the very FIRST need I have for my very first product. It is the root of the business logic of credit, deposit, reservation and layaway. It would be an easier problem if we could ONLY accept multiple recurring payments with incrementing invoice lines.
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Look, I’m new here. My first shopify website is coming up on 4 days old. I have ONE item for sale. If I cannot partial invoice, accept partial payment, deposits increment an invoice and properly track inventory using this system I’m taking my business elsewhere.