Have you read this guide?
https://forum.manager.io/t/creating-sales-invoices/7178
There is no reason whatsoever you cannot invoice services. Thousands of Manager users do so every day. Tax codes are applied to individual line items, so you can mix products subject to one tax code with services subject to another on the same invoice.
You might also like to create non-inventory items for your service billings. I use them on every sales invoice.