The problem is that the script is so poorly documented. There is a header and footer editor included in the script, so they are fairly easy to change, but you actually have to edit the perl scripts to change fonts, etc. If you are fairly comfortable with perl you should find it relatively simple to modify.
Ahh ok. I'm not too comfortable with Perl really; I'd be a lot better with PHP really.
What I really want is just a billing solution.
It's hard to find a good billing system eh?
I started with a list of the following: ModernBill, WHMAutoPilot, ClientExec, PerlBill, Account Manager, Bug-Invoice, WebHost 57, Whois.Cart(), BillThru and 2ndSite - So far I've only ClientExec, Whois.Cart(), BillThru and 2ndSite left.
ModernBill and WHMAutoPilot just did too much. They were on the verge of waking me up at 5am with a nice cup of black sugarless coffee - very nice, but a bit OTT. PerlBill does what I want - however at the moment it's a bit expensive for what it does, and is written in Perl (would've preferred PHP); however if I don't find something suitable I'll put it back on my list. Account Manager is too basic for the price you pay. Bug-Invoice looked awful; however seemed to have a lot of features. WebHost 57 just seems like too much hassle to edit; and also the user interface isn't too good or clean cut.
Whois.Cart() is more than a billing system, however for the price it's very good. ClientExec is very clean cut, and isn't too expensive either - however lacks features for its price. BillThru looks very good, and I'm very interested in it - however it only has integration with Authorize.net as far as I can tell. 2ndSite also looks good and I'm also interested in this - It accepts PayPal, however I'm not sure of which merchant accounts it has integration with.
It, unfortunately, really seems very difficult to find a good billing system which does just that, and nothing else - There is PerlBill, however I'm not too keen on it since it's written in Perl and seems a bit expensive for what it does. Other than that, there's BillThru and 2ndSite - however not much else unless you want an all-in-one-package.