When you get down to a certain level you're not paying for weight anymore, and hence it becomes irrelevant.
Basically, if something's really heavy, you have to pay extra, but it doesn't get progressively cheaper the lighter you get - eventually you bottom out and you're just paying them to take a package across the country. USPS flat rate is a little cheaper than that but they're subsidized (and losing money). If they work with any other carrier that doesn't sound unreasonable at all.
Thats why small little items like this are generally where local shops shine. Even if they charge 2x as much as an online vendor if you avoid shipping its worth it.