If you plan to can your jelly, go ahead and heat up the water bath canner, so the water and the jars within will be nice and hot when you're ready for them.
Add the lemon juice to the flower tea and stir. This will slightly change the color; you should end up with a deep navy blue, purple-ish tea.
Pour the tea and lemon juice mixture into the stockpot.
Measure 2 1/2 cups sugar into a bowl. Then remove 1/4 cup of that sugar. (You'll now be left with 2 1/4 cups of sugar in the bowl. Set this bowl aside.)
Put the 1/4 cup sugar into a second bowl and add the box of pectin. Mix the pectin and 1/4 cup sugar together really well.
Add the pectin + 1/4 cup sugar mixture to the tea and lemon juice mixture that's in the stockpot.
Place the pot on a burner turned to high heat.
Stirring constantly with a spoon or whisk, bring to rolling boil.
Once it's boiling, add the remaining 2 1/4 cups sugar all at once to the pot.
The boiling will stop for a minute. Continue stirring and return the mixture to a full rolling boil.
Boil for exactly one minute, stirring constantly.
If needed, add the butter at this point to reduce the amount of foaming.
After one minute, remove from heat and ladle into jars. You could alternatively pour the hot jelly mixture into a warm pyrex measuring cup and use that to pour the jelly into the half-pint jars.
Wipe the rims of the jars with a damp rag to remove any sticky jelly residue that could interfere with the seal.
If you don't want to can your jelly: leave the jars undisturbed for the next 12 to 24 hours to cool and rest. After that, move your jelly to your refrigerator, where it will keep for about 3 weeks.
If you want to can your jelly so it lasts longer, read on!