ついでがある

expression, godan verb (-ru, irregular)

have occasion to do; have a chance to do

Used when an opportunity arises to do something, often while doing something else. The phrase is a set expression meaning 'there is an opportunity' or 'it is convenient to do something at the same time'.

• 東とう京きょうに行いくついでがあるなら、ぜひ会あいましょう。

  If you have occasion to go to Tokyo, let's definitely meet.

• 買かい物もののついでがあったので、ついでに郵ゆう便びん局きょくにも寄よった。

  I had to go shopping, so I took the opportunity to stop by the post office as well.