I purchased the PDF additional plugin to go with the WP-Invoice, and it wouldn't synch with my install (wouldn't download through their interface).
It took 2 days for Customer Service to respond, after all, they wanted an additional $50 to have them respond 'within 24 hours'. When they responded the first time, they sent me to a link which was already pretty poor on documentation. The next time, they requested that they have full access to my site through my FTP server. Um, no. Sounds Fishy. Do not do business with this group. They haven't charged back the $50 and I am currently in a Credit Card dispute to get my money back.