To skip the thank you page when using a checkout page you, go to:
Left Side Menu Bar > More Features > Checkout Pages
Under "Checkout Pages" page, scroll down to "General" section then click "Edit Checkout Page" button.
Under "Add / Edit Checkout Page" page, scroll down to "General Settings" until you find "Skip Spark's Order Completed Page, and go straight to website URL above?" section.
Lastly, you'll need to click "Save" for the change to take effect.
