Is it possible to require payment for an event before allowing a volunteer to register?
CERVIS does allow for a "Paid Event" opportunity type which uses a PayPal Business account to process payments and requires the volunteer to pay prior to completing event registration. If you would like to have this feature enabled for your account, please open a support ticket by following the instructions in the knowledge base article HERE. There is a one-time set-up fee to enable Paid Events for your account.
Once you have Paid Events enabled in your account, you will need to setup your CERVIS account and your payment account to work together by following the steps HERE.