how can i download an invoice from itunes/apple music?

There are no invoices. You can print the email that came right after the purchase. Or you can take a screenshot from your purchase history:

See your purchase history for the App Store, iTunes Store, and more - Apple Support

Hei! Same struggle here ;) Howeber - check your email, they send you an invoice via email. Click "print" and then "save as pdf" and you got yourself your invoice in a pdf file, that is suitable for accounting purposes.