You can set up default terms & conditions via both the Web Portal and the App, which will be automatically displayed on every purchase order that is created.
In the Web Portal, go to the Settings cog > Accounts > Purchases > scroll to Purchase Terms & Conditions > enter details > Save at the bottom of the page.
In the App, Log in > go to the Settings cog > Purchases > scroll to Purchase Terms & Conditions > enter details > Save.