Good day, froggy-electrica.
Currently, QuickBooks Online App for mobile, including iPad, is unable to create a progress invoice. Though, you can convert a quote into invoice. All you need to do is open the quote, then click the three horizontal dots at the upper right corner. Then, select the Convert to invoice. Once converted, you can also modify the quantity or the price.
I'll surely forward this to our developers so they'll consider and implement this soon.
Another workaround I can suggest is to use your mobile's browser or the Safari. And from there, you can create a progress invoice.
In case you have other concerns, please let me know.