which shopping cart or billing system? - Joomla! Forum - community, help and support

hello,
i evaluating billing systems , getting more confused more dig this. so, if seasoned pros out there dont mind giving me little advice specific product, appreciate it.

product: have produce real estate agent buys physical signs, place in front of house sale. each sign has unique number on it. so, when home shopper calls phone number on sign , inputs unique code know house calling about.

the monthly invoice real estate agent might like:
-------------------------------------
agent abc

call_1 .. unique_code_1 .. call_time .. call_duration .. cost
call_2 .. unique_code_1 .. call_time .. call_duration .. cost
call_3 .. unique_code_1 .. call_time .. call_duration .. cost

call_1 .. unique_code_2 .. call_time .. call_duration .. cost
call_2 .. unique_code_2 .. call_time .. call_duration .. cost
call_3 .. unique_code_2 .. call_time .. call_duration .. cost
------------------------------------

i have of product functionality built , data sitting in sql table.

now, bill service need:
1) generate recurring cost per month
2) generate per minute charge service
3) generate special charges if special flag flipped per call in database
4) promo codes (if run special)
5) volume discounts (a single agent uses 500 minutes instead of 5 minutes in single month 20% off per minute price)
6) group discounts (if 100 agents buy 1000 unique codes under brand of single company, waive monthly fee)

which billing system / shopping cart best suited type of billing? interested in virtuemart because, well, editors pick indicate good. but, seems more geared towards physical products instead of services. true? there better solution type of services, invoicing have described above?

note: running community builder 1.2 (in case matters).

many advice.

regards,
mike