You build things yourself because A: you really like doing that (that is a hobby thing, not something to run a business on); B: you are really good at it and so have made it a core business item you use to make money (you are selling your system to others) C: you can't trust anyone else to do it.
In finance C is a big one! A bad billing system can kill you, and so despite the complexity it might be worth doing just because you now are in control. If you choose to buy a billing system anyway, you still need to understand how it works in enough detail to audit it. A corrupt billing system can hide how it is siphoning your money away and the numbers seem to balance and so you don't realize it is at fault. A bad billing system will not apply some tax it should and when you fail an audit the government will demand you pay - the unexpected bill will kill you. There are many variations on both of the above. You need to audit your systems to ensure they don't happen to you.
Despite the above I generally would suggest you buy a billing system not build your own. However that doesn't absolve you from understanding billing in enough detail to audit yours on a high level. You also need to select one that your independent auditors (which might be too expensive to have but you really want) can audit in whatever detail needed.
In finance C is a big one! A bad billing system can kill you, and so despite the complexity it might be worth doing just because you now are in control. If you choose to buy a billing system anyway, you still need to understand how it works in enough detail to audit it. A corrupt billing system can hide how it is siphoning your money away and the numbers seem to balance and so you don't realize it is at fault. A bad billing system will not apply some tax it should and when you fail an audit the government will demand you pay - the unexpected bill will kill you. There are many variations on both of the above. You need to audit your systems to ensure they don't happen to you.
Despite the above I generally would suggest you buy a billing system not build your own. However that doesn't absolve you from understanding billing in enough detail to audit yours on a high level. You also need to select one that your independent auditors (which might be too expensive to have but you really want) can audit in whatever detail needed.