It happens to us all and it may help to publish our mistakes in order to hopefully prevent others from making the same mistakes.
I shall begin..
My first attempt at re-purposing a Promo bag obtained from the 2014 P&P show:
165c for 20 secs (as per the Metamark 'metatex' guide for Polyester/Acrylic mix). I am unsure of the composition of this bag material :(
I did use my Teflon sheets above and below this - and Two 5mm Mouse mats (side by side) to get over the depth of the bag seams. The Bag Material did not stick to my Teflon sheets and there was no mess apart from the obvious:
.. I have been able to work out through trial and Error that 145c for 15 secs works with this material.