Before ordering the Domain Name, and if it is a new client, check the email address on their cleint account against the PayPal email address
Each case I have had, they have been different
Send an email to the client (not the PayPal email address) that the registration of the Domain Name is suspended as the two email addresses do not match and is being scrutinised against fraudulent use
Also, add details in your knowledgebase with full details
PayPal has improved over this issue recently and if the client does not have the funds in their bank account (the usual cause) you should be notified by PayPal within 6 hours
m8internet.com from Manage My Hosting
Using WHMCS since 11 December 2011, running v5.0.3 since 27 April 2012