I think i'm try to share here .. I've recently tried out some invoicing software
1. kiinvoice - handy but too simple, not able to meet my requirement
2. Sliq invoicing plus - reasonable cool and simple
3. kBilling - rather outdated interface, feature is ok
4. invoiceit! - complicated and outdated user interface
5. Faktun invoicing - reasonable cool and handy but miss out some features
6. xin invoice 3.0 - reasonable cool and features is good
so end up, there are sliq and xin invoice 3.0. After test and play around. Finally choose to pay for xin invoice 3.0. Check out their website http://www.xininvoice.com
just want to share. Please let me know if you have any particular one off payment invoicing software with reasonable pricing.