If the customer is set to pay to a specific bank account, then that will show on the PDF invoice.
If you later change your bank account or change to which account the customer is supposed to pay, then the future invoices will show another account, the new one.
If you are considering to make a client with no user-click, that would not be possible in eAccounting API.
The reason is that we have decided that for our API we will only support authorization code flow.
I can imagine, as a few other of our customers, you would like to use something like "password flow" or "implicit flow" where you can request a token without any user-clicking, but we do not support this.
Some of our customers have experienced that the customizedFields property for customers and contacts endpoints is not working.
If you experience this as well, please contact us and we will make sure this feature is enabled properly for your company.