Good News: Today the MetroBus director announced that ALL of the pay terminals entering MetroBus stations accept all bank cards and credit cards including American Express and also including NFC tap and go methods including Apple Watch (and similar type access).
So, instead of reloading your Metro Card with cash, you can simply tap and go. The reader will simply deduct 6 pesos from your account. But, you can also just reload your Metro Card by using a credit card/bank card too with a larger amount.
Photos below: