If you receive a BitPay invoice, please copy the payment URL. You can then paste this URL into the destination address field when sending from your BitGo wallet. Once you tab out of the Send field, it should auto-populate the invoice amount, and you will only need to confirm the details before hitting send for the transaction to go through.
Please note that we do not support this payment protocol from a v1 BTC wallet so it will not work from there. You will need to migrate to our newer wallets in order to implement this; please refer to this article for instructions on migrating to a new wallet.