Volopay recommends to do it via Paypal, which will be more secure and hassle-free in the this situation. Kindly follow the below steps to make the payment successful:
Create an SG based Paypal account (business or personal one)
Tie your Volopay virtual or physical card to that Paypal account
On the payment site, use Paypal to make the payment
Below are images used to make a payment in Upwork through Paypal and Volopay card:
