Rubber bands are so 2007...
The vast majority of coral farmers use rubber bands for sealing their shipping bags, a tutorial of rubber banding bags for shipping is posted in the Tutorials section by the way. The biggest issue with rubber bands is that if hit hard enough, they will leak. It's extremely time consuming to bag up corals with rubber bands, and doing so for long enough will make for some pretty annoying blisters as well. Fortunately, there's a much better way that's not only faster and safer for the corals, but can save time and money in the long run. My heat sealer saves me about literally 3 hours per day in reduced packing time. Rubber bands are also notorious for leaking, and if your shipping cooler begins leaking your shipping carrier may delay or return the shipment, which will ultimately kill a ton of corals.
Heat sealers, normally called impulse sealers, are made for sealing bags with a thin metal filament covered by a strip of teflon. The metal strip melts the plastic together and creates a nearly impermeable seal if used correctly. Heat sealers are extremely easy to tune to the thickness of your bags, and if your bags are thicker than the heat sealer can handle then simply clicking the heat element twice will seal the thicker bags. Impulse sealers typically run about $20-90, and are about equal quality across the price range. Make sure to get a sealer that is wide enough for your bags, 9" or more is usually a good start. You don't need a heater with a cutting attachment, just a basic impulse sealer, a million of these can be found on ebay usually in the $20-40 range.
Now, heat sealers are not waterproof, and are not meant to be used near water, which is why I'm inserting the disclaimer that if you use a heat sealer around water, then it is at your own risk, especially if it isn't plugged into a GFCI. If you haven't read the GFCI's article in the Tutorials section it makes for a good read, and explains why it's recommended that everything be plugged into GFCI's.
The metal strips in heat sealers will deteriorate over time and need to be replaced, so do purchase a few replacement filaments along with your impulse sealer. Now, there are four screws in the top of the heat sealer that hold the filament cover in place, and these screws are not rust resistent. I went through about 4 heat sealers learning this one the hard way after the screws turned into solid fused rust, so a great tip is to put dabs of liquid teflon for plumbing over the tops of the screws. This will keep water from corroding the screws, which will stop them from stripping the instant that you try to unscrew them to replace the filament.
Once a bag is heat sealed it is immeasurably more resistant to damage, and can take much more of a beating by a shipping carrier than any rubber banded bags. It now takes approximately 3 seconds to seal a coral as opposed to 30-90 seconds or more, and with increased reliability a heat sealer will pay for itself in reduced packing time.