Unfortunately, yes you do need to buy Windows. I am not familiar with Penny Perfect but if it is invoicing software, you won't need full graphics hardware access and a virtual machine will work nicely. I stress that because installing Windows in Boot Camp is actually a complex process. Most of what we deal with here on the forum are those installations gone wrong. Virtual machine installations, on the other hand, are much simpler and have fewer issues overall. But it's really your choice.
VirtualBox is software you install on your Mac first. That software will create the virtual machine for you once you have a Windows disk ready to go. There are other options for virtual machines (VMWare Fusion and Parallels Desktop, for example), but they cost about $50+ USD so don't meet your "cheapest" criteria. I use Fusion (and prefer the interface) but they all work well.
Or you can do boot camp. If you choose to boot camp, definitely print out the Apple Boot Camp guide (online at Apple support) and follow it TO THE LETTER. Parts are not intuitive. You have been warned ...
And either way, please read the rest of the posts in this sticky and the other one on this forum. Both will give you most of the info you need to do this succesfully. Any specific issues, come on back.
One last thing: on any Windows installation, you will need to install an anti-virus program. My best suggestion currently is Microsoft Security Essentials. Free and speedy, it does the job. Other free options are AVG Free but its gotten a bit bloated in recent years. Still works ok. The good news is that if you pick up any viruses on your Windows installation (either Boot Camp or virtual machine) they won't affect your Mac OS side of the equation.
Good luck