Is there a way I can set a default discount for a certain client?

Yes, you can set a default discount via the Client or the Project.

First, go to Clients in the navigation bar, then from here click on a specific client. You may also go to Projects (or whatever your mnemonic is for this term) and click on the specific project you'd like to set this to.

From there, click on the clients' or projects' Invoice Presets tab and click on the blue Edit button. 

You will see the Invoice Discount field to the left, which you can either set to Percentage or Flat Fee. Enter in the amount and also set a default to apply to Labor, Expenses, or both. Then click Save when finished.

If you are setting this up for a Project, please make sure you uncheck the "Use Client Settings" box so you are able to fill in the those fields. 

Note: This will not update previously created invoices.