Honestly, why bother?
Use an ERP software or accounting software with pre-loaded information on all countries and let it generate custom invoices.
Link the ERP system to your shipper (DHL / FedEx) and let it upload the PDF file or just print 3 copies of the invoice.
Where is the Issue?
Shopify should add inhouse sorting and stackable discounts, as this is much more a pain
OR UK 135 Pounds threshold.