In the settings of sales orders you can upload attachments. This allows you to upload general terms and conditions and send them by default along with your sales orders.
1. Open the Sales Orders submenu of the Orders & Deliveries module. Then click on the Sales Order Settings.
2. In the basic settings, go to "Attach files". Here you can now upload attachments as general conditions. Don't forget to check the attachments as "Default" to always send them along with your sales orders.
3. If you don't do this, you can also add the files manually when sending sales orders by email.