Laser transfer paper takes a lot of heat to transfer. You need to use high temp mask such as Magic Mask. Otherwise, the mask will melt on the shirt if it can't take the heat.
Why don't you try Imageclip for laser instead. It is self weeding. Don't need to trim it.
Works well even on light pastel and slightly darker color shirt.
Or you can try the masking technique that I use for inkjet or laser transfer paper that leaves polymer window. I use plain paper that I contour cut then use it to cover the unprinted area of the transfer paper.
Check this link:
http://www.t-shirtforums.com/p448921-post12.html
BTW you can get the carrier sheet to hold the transfer or plain paper for contour cutting from Joann's fabric. A lot cheaper than sold by vinyl supplier. You can't you use it for pressing.