Hi Greg,
If the fields you added to your Customer table are static for the customer (i.e. they do not change per estimate / invoice / etc.), then the customer table is the best place for them.
You can add these fields to your Estimate table as Linked Fields by clicking on the Estimate table from Customize > Tables / Fields in Method. You'll want to link them using the Customer dropdown.
The only downside here is that they won't be editable from the Estimate, but there are ways you could get around that. May I ask for a bit more information about the fields you're referring to, as well as why you want to edit them on the invoice?
- Justin