I found a workaround in QBO itself. I'll try my best to lay this out in a way that make sense:

1. run the report called "Sales by Product/Service Detail"

2. click on "customize report", then add and specify the following filters:

    a. check "customer", then select the customer for the invoice you're trying to export to excel.

    b. check "transaction type", then set it to "invoice".

    c. check "num", then type in the name of your invoice.

3. click on "run report".

4. the report should then show you all of the individual items and their quantities on that invoice, and on that report there is an export button at the top, where you can export it to Excel. I would imagine there's a way to run a similar report for bills and purchases, but I haven't gotten that far yet. Maybe someone else can try to follow my same thought process and figure something out there.

This is the closest thing I've found to being able to export an invoice. Once exported, you can delete the columns you don't need shown, and this should give you at least a good head start for what you're trying to do. I had the same question as OP, and after seeing Intuit's response saying there was no way to export an invoice, I did some playing around and happened to discover this solution. Hopefully this helps everyone.