I have been charged USD1 by PayPal. What is this for?

This is a temporary hold that PayPal occasionally uses to verify a new credit card. PayPal holds this amount and soon releases it after verification.

You can read more about this procedure at this link:

https://www.paypal.com/sg/webapps/helpcenter/helphub/article/?solutionId=FAQ463&topicID=&m=ARA