No shade whatsoever at you or your business: I'll say upfront that you certainly made the right practical decision for the goal of running a business.
That said, this is a textbook example of what I have always found so infuriating, personally, about working on commercial software, and one of the many reasons I ultimately moved into a non-software-writing role. The (very sensible and practical) shortcuts and tradeoffs that are commonly made due to time and cost constraints. The attitude of "well the vast majority of our use cases work, so we're done." I've always thought edge cases must be addressed. Something in my brain hurts when I knowingly release something where only 99% of cases work.
I can imagine this is probably the same thing some artists feel when they are commissioned to produce (in their view rushed, flawed, or incomplete) artwork for business purposes.
I only write software at home, as a hobby now, and this gives me the outlet to follow my heart around edge cases!
imo it's not a great solution. the problem is there is no standard or source of truth for zip code boundaries because they are a usps concept used for mail logistics. zip codes change all the time, are approximations of an area, and generally shouldn't be used for something that requires precision like calculating rates. may be ok to use as a fallback though.
also i hear your point on swe roles and don't disagree