Invoice Total from other App as Line Item

Hello,

We use a custom made app for medical records, which then creates an invoice. Through API’s, we have then been transferring that to Quickbooks for in persona and online checkout. I’ve been trying to figure out a way to get this accomplished within Shopify. That invoice is often complicated (service fees, situational based product pricing, etc.) that Shopify doesn’t support, so adding all of these services as products in Shopify won’t work very well.

Is there a way to import that invoice total into a ‘custom item’ and then have some way to display the detailed invoice from the other app? I can’t find any way to attach PDF’s or external links to Shopify invoices/orders.

Does anyone else have any possible ways to accomplish this?

Any ideas or suggestions would be greatly appreciated. Thanks for your time.