expression
while you're at it; take the opportunity to
Used when doing something as a natural extension of another action, often with a sense of convenience. Frequently appears in the pattern 〜のついでに or 〜ついでに. The expression 物のついで is a set phrase meaning 'since you're already doing one thing, you might as well do another.'
See also: 事の序で
買い物に行くなら、物のついでに郵便局にも寄ってきて。
If you're going shopping, stop by the post office while you're at it.
掃除を始めたから、物のついでに窓も拭こう。
Since I started cleaning, I'll wipe the windows while I'm at it.
A more common and flexible conjunction meaning 'while you're at it' or 'incidentally'. 物のついで is a set phrase that emphasizes the idea of taking advantage of an existing situation, often used as a noun-like expression followed by に.
A similar set phrase with the same meaning, but less common. 物のついで is more frequently used.
From 物 (thing) + の (possessive particle) + ついで (opportunity, occasion). The word ついで originally meant 'order' or 'sequence', and by extension came to mean 'an opportunity arising from a sequence of events'.