Retail x eCom sync

I am a long-time Retail user and I am setting up a new webshop with eCom
Some questions that pop-up:
- - customers from retail are not being synced with eCom and another way round. Why? Can we get this done?
- - Invoicing: If I do a sale in our shop (with retail) and a customer asks an invoice, how can I generate an invoice via eCom?
Thanks!
Bart