Prepare the jars by running them through a sanitize dishwasher cycle, or boiling them in a pot of water for 10 minutes, then keeping them in the hot water until needed.
Add the 2 cups of berry juice to the pot.
In a small bowl, combine 1/4 cup of sugar with the 1/2 pack (25 grams) of low or no sugar pectin.
Stir the pectin/sugar mixture into the berry juice.
Place the remaining 1 1/4 cup sugar near the stove, so you can grab it easily when needed.
Place the pot of juice over a burner turned to medium-high or high heat.
Stir constantly until a hard rolling boil is reached.
If the jelly starts to foam, optionally add the 1/4 tsp butter to help reduce or eliminate the foam.
Once boiling, dump all of the sugar in at once, and stir to dissolve.
Return the berry juice to a full boil, and boil for 1 minute.
Immediately remove from heat and pour into the hot jars. You may find it easier to pour the liquid jelly from the pot into a 4 cup Pyrex measuring cup first, to make pouring into the jars easier.
Cover with lids and leave at room temperature until cool.
Store in your refrigerator for 2 to 3 weeks. Jelly can also be frozen for about 6 months, though sometimes the texture may change.