Merchant can now request to accept payment for USD by doing self activation through their Xendit Dashboard, aside the default PHP once they press activate cards payment method. See radio buttons as shown below on the sample activation form and make sure to click on YES.
Given this, activation of your USD MID will take 7 - 10 business days for processing and for it to be activated. Once it is, merchant can already proceed to transact using USD as currency.
Once your USD MID has been activated. You can configure this from your Shopify admin. Go to Settings > General > In the Store currency section, select the preferred currency. Click Save.