The 18-year-old Santiwiwatthanaphong birdied the par-5 18th for a 5-under 67 and a one-stroke victory.
"I'm very happy to win this tournament," Santiwiwatthanaphong said. "It means a lot to win the last event and win my first event in the United States."
She had a 14-under 274 total on LPGA International's Jones Course and earned $22,500 to finish 20th on the money list with $40,614. The top 10 on the money list earned LPGA Tour cards.
Do closed with a 69. The former UCLA player earned $13,840 to jump from 12th to third on the money list with $64,877. Last week, she also finished second to go from 18th to 12th.
"I'm overwhelmed with all kinds of different emotions," Do said. "I'm excited. I'm relieved. It's just a crazy moment right now and I'm trying to soak it in."
Annie Park topped the money list with $68,673 in only 11 starts. The former University of Southern California player won three times and swept the player of the year and rookie of the year awards.
"It has been a lifelong dream to play on tour and to finally get status is unbelievable," Park said. "I still can't believe that I got my card, but I'm going to have to believe it next year."
Giulia Molinaro was second at $68,632, followed by Do, Daniela Iacobelli ($62,692), Dani Holmqvist ($62,160), Lee Lopez ($61,731), Alejandra Llaneza ($61,289), Vicky Hurst ($57,814), Casey Grice ($55,621) and Rachel Rohanna ($53,867).
Rohanna edged Haruka Morita-WanyaoLu by $284 for the final LPGA Tour card. Augusta James, ninth on the list entering the week, dropped to 12th. The Canadian missed the cut.