We don’t really have any provision for storing Ts and Cs, or anyway to accept or decline them. You could either add a link to the payment terms on your invoices, something like “By accepting this estimate you are accepting the following terms and conditions”
Just enter something like this in your payment terms box:
By accepting this estimate you are accepting the following <a href="http://support.quickfile.co.uk/terms_and_conditions.htm" target="_blank">terms and conditions</a>
The other thing you could do is attach a file to the estimate containing your Ts and Cs and again refer to them in the payment terms or notes box.