i currently use modern bill as a merchant system and im very happy with it but i had an email from one of my clients asking what good free billing softwear is there and i didnt really know of any so i decided to post it here

There's nothing free about e-commerce. It really depends on what type of "billing" your client needs to do. PayPal can be used for billing and doesn't have any setup or monthly feesbut there are per transaction fees.

i ment billing /merchant systems