Bag Closing with a closure may be with either a wire twist tie or a piece of tape. 

Tape:

Bag Sealer with tape.  Good for low production but your customer may have difficulty re-closing the bags with the same piece of tape.

Twist Tying:

The second method of closing the bag is to use a Wire Twist Tie Machine.  Similar to the twist tie found on loaves of bread at the grocery store.  Twist tying machines are available in 3/8", 5/8", 7/8", and 2" models.