Is it required to send the card's CVV in each purchase request?

It is up to your provider whether it allows processing transactions for existing cards without sending the CVV again with each purchase.

You can learn whether your desired provider enables this feature in our developers' documentation, under the Provider Integration Guides. Simply look for the "Transaction Processing without CVV" under the Features section in your provider's integration guide.

If your desired provider does not allow this feature, you'll be required to attach the CVV value with each authorization or charge request.

However, if your desired provider does enable this feature, you need to attach the CVV value to the payment method's first transaction with PaymentsOS. Once the card has been verified the first time with its issuer, the next purchases with the same payments method will not be required to include the CVV value.

You can read more about it at Collecting the CVV Code.