The text of the emails you send using LSP.expert can be changed in Settings > Document and email configuration > Emails tab of the configuration in question. You can format it as you like (bold, italics, etc.) and define a different text for your invoices, invoice reminders, quotes, purchase orders, etc.
You can customize the content of your emails using variables. When you click on "help on variables", you see all the variables you can use:
You can insert those variables anywhere you want in your emails and the system will adapt them with the correct info when sending your email.
You can also add up to 2 default attachments (your terms and conditions, for example) to the emails you send with LSP.expert, by clicking on "Upload an attachment" (see screenshot above). These attachments will be automatically sent with all the emails you send through LSP.expert.
When you're done, click on Save.