How do you have it automatically apply to invoices how many days payment is due without adding at each time?

Thanks for reaching out to the Community, cataldi22-gmail-.

You can set default invoice payment terms from your Account and settings.

Here's how:
 

1. Use the Gear icon, then go to Account and settings.
2. Access your Sales tab.
3. Find the Sales form content section and click its Pencil icon.
4. Specify your default terms in the Preferred invoice terms drop-down list.
5. Select Save.
   
   

Once you've saved your changes, the terms will be automatically applied to all new invoices moving forward.

I've also included a detailed resource about working with payment terms which may come in handy moving forward: How to Adjust Invoice Payment Terms

Please feel welcome to send a reply if there's any additional questions. Have an awesome Monday!