Attach Doc to ShipStation Order

I use ShipStation to connect my 3PL to Shopify for order fulfillment. I drop ship orders for several marketplaces (Faire, RangeMe, Maisonette) that I sell through which require that I attach custom invoices. I'm looking for a simple "attach PDF" functionality that could live, for example, in the order notes section. It would look a lot like the link at the bottom of this word box that says "attach a file". It would allow me to associate packing slips with orders for my 3PL. The workaround for this is so brutal that I may have to leave shipstation.