Actually you don't have to add funds directly to your cards!
When you create one or multiple Airwallex Borderless Cards, they will always be funded directly through your Wallet Balance. This means that if a card payment is getting declined due to insufficient funds, you may have to top up your Wallet in order to sufficiently fund your card(s).
Currently Airwallex cards support direct billing in 10 different currencies. Payments in these currencies will be pulled directly from your corresponding Wallet balance if available. If you have auto-conversions turned on, then funds will be pulled from your Home Currency if you don't have the required balance in the direct billing currency in question.
Please see the following article in regards to direct billing currencies for your cards and auto-conversions: What is the automatic conversions for cards?