Drop whatever dry soup mix you have into a burlap sack. And since hessian cannot be heat sealed, you will have to sew the sack with your hands. Alternatively you can use a duct tape to securely seal your soup mix.
The permeable texture of burlap sacks makes them an ineffective barrier against moisture. If you intend to store your dry soup mix for a long time, it would be better to change your strategy.
A resealable stand up bag forms an effective wall to protect your dry food from moisture. A way to store your dry soup mix for a long term is to package the mix in a dry stand up pouch. You can then use a zip lock or another accessory to seal your bag. If your bag is not resealable, customers would have to store the dry mix in another container. This is generally not recommended. With a zipper, your pouch becomes multipurpose and your customers would prefer your product more than your competitor’s.
Unless you have an emotional attachment with hessian burlap, you can package your soup mix in a resealable pouch bag just as well. Stand up packaging will keep your soup mix dry and protect it from environmental hazards better than burlap sacks.