Hello there, @Lise46.

You’ll want to add a payment term that is due within 30 days from the invoice date.

Here's how to do it:

1. Go to your Lists menu.
2.  Click on Customer & Vendor Profile Lists and then Terms List.
3. Click the Terms drop-down and choose New. 
4. Provide a name for this term and then choose Standard. 
5. Select OK.

Check the following article for more information about this: Set up payment terms. 

Please let me know if you have any other issues or concerns, and I'll get back to you as quickly as possible. Have a great rest of the day!