Tried calling Hostbill phone with no success, sent a custom development email without any response yet.
It's very hard to get hold of him both pre- and post-sales.
We need several invoicing and payment tweaks and have no idea if its possible.
Depends on the tweaks you need. You can technically add your own hooks, plugins, template files, etc... but seeing as there is virtually no documentation on the classes, functionality or inner workings you are essentially blind if you don't buy the source-code version.
Does the platform works well with postpaid customers ?
While the system supports postpaid invoices of sorts, it really is not designed for such usage scenarios and is really designed to fully automate prepaid billing handling as that is the most prominent in webhosting environments.
I have to say that it does handle prepaid very well with lots of options and I consider prepaid superior to postpaid for such services both for the operator and for the customers.
And last but not least, is there support service good?
You are unable to contact him for pre-sale support. Do I really have to answer the question how support will be post-sales?

To be fair the software is very good if you get it to work the way you need it and it works rather reliably. However it keeps having regression bugs in about one of four updates which are usually quickly fixed but can cause major disruptions like proforma invoices being sent out without an invoice number. Testing is obviously not one of his strengths.
We have been evaluating WHMCS
Imho WHMCS is terribly bad. It looks nice, works nice but no matter how much time I spend, it never seems to actually do what I want it to.
im looking for a robust system with reliable contact and tech support from the vendor
If those are your criteria (and I well understand them) then Hostbill is currently NOT for you. However I can't recommend an alternative either because a lot of the alternatives we evaluated when switching to Hostbill in 2012 are either dead, bought up or otherwise now uninteresting.