Membership 2 Pro billing problem: issue with PayPal Sandbox

I'm testing a brand-new installation of Membership 2 Pro. I have configured some paid Dripped Content Membership tiers. When I try to check out using the Paypal Sandbox I get the following error:

"This invoice has already been paid. For more information, please contact the merchant."

The error indicated that this invoice ID has already been processed in the PayPal sandbox. How can I generate a unique invoice number? Am I able to prefix the invoice ID with a unique string?

Thanks,
Ciaran