Here's from a discussion in 2010 about Mylar with information about it.

REAL Mylar is NOT for embroidery; it is a polyester film invented and registered by DuPont--if it is not manufactured by DuPont or its subsidiaries, it should not be called Mylar. Somehow the name has been hijacked by the embroidery industry for a similar, unequal, product.

The iridescent film is used to add sparkle to your designs. It can be used as a filler to FSL; however, it is best used with designs specifically designed for its use. It is not intended to be used on items that need to be washed or ironed. However, I did send a 4 in x 4 in square of Voila Tissue Wrap (from the Dollar Store) through the washer and dryer with satisfactory results. Hope this is helpful to you.

I have used mylar from the dollar store with success. The key to me is using a design digitized for use with mylar. That makes all the difference. Hugs. Nan W

when you look at Charmingstation.com and click on the left on Mylar Designs, you will find how to's and lots of designs. Hope this will help.
Hugs, Martha

I use the $ store voila for fsl , but bought some opalescent white from Heirlooms by Sharon for some of Charming Station's snowmen in my son and dil's tree skirt. That might need a bath at some future date.
Jo