If you have a Bus ticket for a destination abroad, i. e. to Phnom Penh or Padang Besar you should be fine. Yours ends in Thailand and will most probably not accepted.
With Passport Card you don't need to pay upfront, but they are pretty expensive. Most others, if not all, reqiere your payment upfront to the hospital and refund it later. Most, if not all hospitals are willing to contact your insurance upfront and let them send a guarantee in more expensive cases, but you will have to wait for the answer and that might be a issue in emergency. The hospital in this cases will charge your insurance directly. This is about real international insurances, not some travel plans.