Just a heads up to anyone with wp-invoice problem, getting the following in an email field:
or the following on PayPal:
Return to Merchant
We cannot process this transaction because there is a problem with the PayPal email address supplied by the seller. Please contact the seller to resolve the problem. If this payment is for an eBay listing, you can contact the seller via the "Ask Seller a Question" link on the listing page. When you have the correct email address, payment can be made at http://www.paypal.com.
These are probably caused by a conflict with an email obfuscator.
For example, I was using wp-recaptcha and encountered this problem. Not fun when your clients are ready to pay!