There is just enough of a strip above the "ziploc" line that you can heat seal it. I've been too lazy to buy a real heat sealer so I just carefully use the edge of a clothes iron. Medium heat, press and move slowly. You can see when it starts to seal. I go over it a couple times to make sure.
I think
@Mañ'O'Green has said you can seal the bag multiple times all the way down.