名詞
順便幫人哋一個忙 a favour which is convenient to carry out since it fits in with one's plans; literally, a favour with the flow of water
反正我都整緊呢類袋,幫你整多個都順手,當賣個順水人情俾你啦。
反正我都整紧呢类袋,帮你整多个都顺手,当卖个顺水人情俾你啦。
faan2 zing3 ngo5 dou1 zing2 gan2 ni1 leoi6 doi2, bong1 nei5 zing2 do1 go3 dou1 seon6 sau2, dong3 maai6 go3 seon6 seoi2 jan4 cing4 bei2 nei5 laa1. I'm doing the same type of bag anyway, just do a convenient favour for you to make you one more.
Definitions (Wiktionary)
phrase, idiomatic
favour done (for others) at little or no cost