The only way to combine an invoice and courier label is via an A4 paper size method. A4 paper and the toner are costs that most businesses within e-Commerce have eradicated as much as is possible. So a more cost effective way is required to print the courier label and invoice.
You can already do this by creating a custom despatch note on 4x6 (it says 4x8 on the dropdown but just use it!) and then assign this custom despatch note to be the default note for all orders (or by channel is you need to). I've got this running for 50+ clients - works like a dream. When the packers start scanning a product from the batch, the despatch note spits out of the Zebra and when the order is complete, the courier label comes out.