N3
ついでに
While you are at it, On the occasion
買い物に行くついでに、郵便局に行ってこの荷物を出しておいてくれる?
Can you bring these packages to the post office while you go shopping?
Explanation
「〜ついでに」 means 'while at it'. You go somewhere for a main purpose and, taking the chance, do a small extra thing that costs little effort: 「郵便局へ行くついでに、スーパーで晩ご飯を買った」 (On my way to the post office, I also bought dinner at the supermarket). The first part is the main errand, the second is the side task. Attaches to noun + の, dictionary form, or ta-form.
Mixed up with
「〜がてら」: both actions count about equally, two birds with one stone: 「散歩がてら買い物する」 (shop while taking a walk).
「〜かたがた」: very formal, for visits and thanks: 「お礼かたがた伺う」.
Do not tell a superior or guest 「ついでに寄りました」 (I just dropped by on the way). It sounds as if they are not important.