Tim S, you are right, even I have designed systems like yours.
But - and a big but, you really don't see the logistics behind that, so simple as for example taxes, varies by source, destination, product, etc - even worse pro-rated changes, changes to when sent or received, tolls, whatever. Now, add to that the currency changes, fluctuating daily or maybe hourly, non-payments, disputes, even returns (rules change, you know) and maybe changes in accounting laws, regulations, etc - again maybe pro-rated. And because you are VAR (or whatever), all these work on both sides, both on your providers and on your customers. Find me a software package which can handle all those including your house rules without a lot of re-work.
Believe me these were just the simple examples, systems change, infrastructure changes, vendors go out of business, service contracts fail every day, business grows and needs more resources, new laws are created which can change almost anything, knowledgeable people change jobs, get sick, retire, even die - your system is built for one OS which gets obsoleted and the vendor doesn't care, your system only supports one type of interfaces / browsers / etc which are found to be unpractical / unsafe / etc, the management wants to add new business functions, get new types of reporting, connect to new financial interfaces / institutions, whatever.
The current problem with ERP or whatever systems is that they are not achitected to (your) business needs (most often, not a rule) but try to force the business to follow the ways products work. These can be easily dealt when you design a system from your business perspective instead trying to force your business to match whatever is the current fad.
Some business is so volatile (dynamic) that I sometimes wonder (not much, another story!) how they can even think some outside system provider knowing better than they what they do / need?