Mega Bold Gel Pen by Promarx

The two pack is available at Dollar Tree, I only saw black ink at my local store but it's possible they might also be available in blue 

The ink is super juicy, not the best to use on the fly or on thin paper but looks like the ideal on mailing envelopes for that extra bold look. Note: the ink needs to dry (longer than regular gel) before it's smudge proof.