Cairns came fourth in her K1 (one-person kayak) 500 metre heat Thursday.
Similarly, Tuva'a Clifton earned the quota place for Samoa after coming third in his K1 200 metre heat Saturday.
A fifth-placed finish in the semifinal put Clifton in the overall B Final, where he came third.
Cairns missed out on the K1 500 metre final after coming eighth in her semifinal Sunday.
She also ended up placed ninth in the K1 200 metre B final, and competed with sister Bridget Cairns in the C2 (two person canoe) 500 metre event.
Clifton came in third in the K1 200 metre B Final, with fellow Samoan Rudolf Williams missing out after finishing eighth in their semifinal.
The two paired up for the K2 2000 metre event, where they ended up seventh in the B final.
Clifton, Cairns and Eroni Leilua join weightlifter Don Opeloge and discus thrower Alex Rose in Team Samoa for the Tokyo Olympics which will be held from 24 July to 9 August 2020.