Free Freight For Vouchers

Select "exclude normal frieght rules" on the product freight rule

1. Edit product
2. Pricing Tab
3. Freight / Weight
4. Freight Rule =  "exclude normal frieght rules" 
5. Freight Domestic = 0
6. Freight International = 0

That should be sufficient. There will be no freight cost associated with any line item that has this setting. However, if any product in the cart is not excluded, then the normal freight rules will apply for the cart as a whole. 

Irrespective of excluding normal freight rules, the system will attempt to identify which region the customer is from, and apply a freight rule by name. However, it won't apply the fees if all items are excluded from freight rules. 

Another Approach 

1. Edit your voucher product, and assign them to a category called "Vouchers"
2. Navigate to shipping and freight management
3. Create a freight rule with no cost called "Vouchers Only" 
4. Product/Order Filters - If all products are in the category = "Vouchers"
5. Optionally edit your other shipping options and add a filter of: Not if all products are in the category= "Vouchers"

In this approach above, if all the products are vouchers, then this shipping option will be available for use, and as it will be the cheapest shipping option, it should automatically apply to the cart. The last optional filter helps ensure no other shipping options are presented. 

If you apply both of these approaches, to make your products exclude shipping fees, and to also hide irrelevant shipping options, then your users will have a better checkout experience. 

Notes: If shipping rules exist, then at least one rule must apply. So hence there needs to be a shipping rule for a cart of vouchers, even if the products do not require it. Simply to ensure that the cart does not fail with no shipping rule chosen.