To seal an envelope close if the sticky part underneath the flap is not holding, use a glue stick instead of taping it down.
In case you haven’t noticed, adhesive tape has some “weight” of its own. Using it to seal an envelope that refuses to stay closed may add to the total weight of your mail. More weight means you’ll have to pay more, and no one wants that if they can help it.
An exception is if you intend to use decorative tape (such as Masking Tape -@mtmaskingtape) to hold the flap down. That merits a different entry in our toolbox altogether.