Thanks for your question @...! Here are a few guides to help you out:

• Add Terms and Conditions to your Invoices
• Add a Discount Line to an Invoice - Keep in mind if you're syncing to MYOB or another 3rd party software this method won't work, you'll need to discount an individual item on the invoice as shown in the next guide
• Add a Discount to a Line Item on an Invoice

What it sounds like you're aiming for is a fully automated way to reduce the price for an invoice paid early. While this isn't currently possible in that way, what you can do is add a discount by the methods above, and then remove it when you get the reminder that the invoice is due.

Alternatively, you could add it while accepting the payment from the customer, although if they are paying online you would need to coordinate with your customer a little more.

If any other users have a system for an early payment discount, let Greg know in the comments!

What I want is to assign invoice payment terms by customer. These terms would prefill when the invoice is raised. This is the way all the other systems work. Trying to remember all the terms for all the customers is not possible. 

Aha, that makes sense! Unfortunately that is not a feature currently offered with Business Center. This is great feedback, and we will absolutely add this request to our feedback center so other users can vote for this feature to be created!