These are what we use for commercial & industrial bagging.
These nylon zip ties are 57-lb test and provide an easier & faster way to secure bags than using the sometimes-flimsy ties that come with all of our bags.
For earthbag building, these aren't really necessary. Simply giving the neck of your filled bag a good twist & securely tucking it under the bag as you place it horizontally is generally enough. Make sure you don't get careless if you do this - if your earthbag fill is wet, it can spurt out under the impact of tamping, or it can slowly ooze out. Make sure to keep your mud in the bag where it belongs.