you can always buy standalone PA which is renewable on annual basis. very cheap premium.

actually Mediexpress is not an insurer... they are merely a Managed Care Organisation (MCO in insurance term). they have the network that manages the medical card. the actual paymaster to whom you pay your premium and from whom claims are paid out are the respective insurance companies or called underwriters.

many insurers like rhb, amassurance, maa and allianz relies on MCOs to manage their clients card system, to minimize cost. specialised companies like ING (previously Aetna), Prudential and AIA on the other hands are each a very large and specialised group with their own in house department that perform the MCO function.

in fact Aetna used to offer their Genesis (Aetna's subsidiary MCO) cards to be used by other insurance companies, but have since stopped just before they become ING.