Yeah, me too. I wish I could do everything in the app vs having to use the site and the app for different things. Oh we'll, maybe in the next version?
I can't find a way either however you will be send an email from ebay informing you that your item has sold and within that email there are various options one of which is 'send an invoice'.
What every one else is saying: you can't do that from the app. You have to go onto the website on a PC.
Unfortunately, there is no way to issue an invoice directly from the app. Maybe eBay will make arrangements to include this feature in their next update.
Thanks for the info