All the best. As a salesforce admin and consultant I came across JustOn in recent projects. After digging into the tool and setting it up for our customers all I can say, great.
It brings a whole bunch of features out of the box, can be configured pretty individual, besides invoicing has good accounting features to prepare your data for e.g. DATEV. 
If you don't know what to do, the support is in place very fast with very good advice and help. 
These people really do a great job, extra probs! Up until now I did not come across anything which could not be solved when it comes down to invoicing.

Mano von Peak & Peak