This here is what worries me! I have no problem sending in some sort of identification, however I am totally uncomfortable with sending my US passport in for verification.
They did not indicated any other foreign's ID but if sending your passport worries you, there's no harm of asking their support. Prolly they haven't mentioned any option for foreign because locals are more of their priority. As suggested by harizen, you can ask their support if there's any other ID that you can send to them for verification.
Other way of contacting them is through their email:
help@coins.ph and/or
support@coinsph.zendesk.com