I have been using sae for 20 years.
I use self seal ones, and have a stamp that prints the following.
Name Date
Your bill is £ : Round
Made payable to [ your name ]
I lift the flap of envelope and stamp this under the flap, I do about 30 at a time. Then all you have to do is fill in the spaces.
I have never put stamps on and have a 99% return rate.
Roy