Just do On Arrival. You have options to choose 15, 30, 60, & 90 days. Once you land at TIA, go to the KIOSK and apply. Make sure you have a smart phone to take picture of the confirmation page as KIOSK doesn't print the receipt as they mentioned on the immigration website! This is what you show at the counter to pay the amount. You can also fill the application online before departure if you don't want to wait on line at the KIOSK. If you do, you will need to print the confirmation page with the barcode and you can directly go to the payment counter once you land at TIA. Choice is yours.

Thanks!