I have a Food Saver with the seal only feature and have sealed treat bags, potato chip bags, cracker sleeves, rice bags, brown sugar bags... I just don't let it seal for the "standard time".
I just tried a "treat bag" (which was pretty thin...I got it at Michael's). I counted to 5 (not slowly, so maybe 3 seconds) and flipped the latch up to stop it and raised the lid and removed it immediately. Perfect seal, no bag melting. I have melted a plastic bag before but just moved down a tad (it was a bag of rice I think) and sealed again for just a brief couple of seconds. Works beautifully to keep all of my pantry items from going stale in our incredibly humid environment. One thing I also have noted from experience...the seal bar on the Food Saver, after using repeatedly, gets hot...so reducing the seconds of seal time as you do larger numbers would be a good idea so as not to get to the melting stage. Hope this helps!