To send an invoice with your android phone you will need to log on to the eBay desktop website. This is the site you would see if you were on a computer.
Then go to your Orders page and click on one of the items sold to a buyer with multiple purchases. You should see a tab that says "send invoice" and click on it. An invoice will generate automatically.
In the invoice you have several options. You can discount the sales price or shipping charges (or increase shipping or add an additional fee).
After you make the adjustments you want simply hit the send button and your buyer will receive the invoice within minutes.
I use the desktop site on my phone a lot. It gives me so many more options and tools than the app.
Hope this helps.
