Thanks for your suggestion about having a local fallback for USPS rate lookups. I've discussed it with Development, and they're looking into it; if it turns out to be feasible and fairly straightforward to implement, they will do so.
From what we've discussed so far, if USPS fails to respond when we query for a rate, or responds but returns no usable rates, then we would resort to our local fallback lookup, but if they return at least one rate for any USPS shipping rules applicable to the buyer's order, we would accept that.
It's normal for USPS to not return any rate at all for shipping method(s) that aren't available for the buyer's order weight or destination, so we can't tell when that's the case vs. some glitch causing the missing rate(s). This means when our fallback comes into play, there's a slim chance this could result in applying a rate that isn't actually available for the buyer's order weight or destination; however, we reckon it's better to at least let the buyer place their order and sort out any potential shipping-rate issues later if needs be, rather than to prevent them from ordering at all.