Q: I need to produce invoices on ipad need an app that is reliable?

You can only buy apps from Apple's app store, so they are all equally "recognized."

Apple's own app is Pages.  Have you tried that?   If you have tried that or others, tell us, so we don't waste time suggesting things you don't like.

Both Apple's Pages and Numbers have templates for invoices.

If you have a microsoft account that is linked to an office subscription, then you could also try Word or Excel.

You do realize you are responding to a thread that is two and half years old, right?

Yes. The reason I replied is solely for the help of other people. I have had help from a thread that was 3 years old before, they still help!