Payment Conditions and Additional Notes will appear in the invoice template when you use the keys below:
{InvoicePaymentConditions}
{InvoiceAdditionalNotes}
The default payment conditions and additional notes text you can set up in Property Settings / Financial Settings / Invoice Settings:
The text can be modified every time you create a new invoice:
In the default template, this information will appear at the end of the invoice but using the keys you can create your own template for the invoices: