A package gets accepted at the post office counter, ships without a problem, and weeks later the shipper's account takes an adjustment: a $50 hazardous materials noncompliance fee. Nobody warned the shipper. The counter clerk didn't catch it. The parcel was already delivered.
I kept running into this story while researching USPS's dangerous goods rules, and the more I dug into the official documents, the stranger it got. So I built a tool to catch it before the label gets bought. This post is about what I found and how the tool works — the design might interest you if you build anything where correctness matters more than cleverness.
Here's the fun part. The authoritative sources don't agree with each other — and the newest information isn't where you'd expect it. On 2026-05-15, a Federal Register document (2026-09785) described the fee as an 8-point "Hazmat Quality" check with a 100% threshold. Seven weeks later, USPS walked that back in a PostalPro Industry Alert (2026-07-07): the fee is assessed per package through Census and Automated Package Verification (APV), by comparing four data points — Service Type Code, service icon, barcode banner text, and DOT markings. The same alert says a package that both skipped the handling fee and failed the markings check gets charged twice. The Federal Register text only says the higher of the two. And the bit I couldn't find in any Federal Register document at all: USPS Returns shipments become subject to the fee starting 2027-02-01. That date exists only in a follow-up Industry Alert, corroborated by Postal Bulletin 22709 (a DMM revision).
So the ground truth lives in three Industry Alerts, a Federal Register notice, and a Postal Bulletin — none of which a regular seller would ever think to cross-reference. Meanwhile the top Google results for "why did USPS charge me a hazmat fee" are forum threads and outdated blog posts.
That gap is the whole product thesis: the answers exist, they're official, and they're scattered across documents that contradict each other.
CanWeShip is a free pre-shipping checker: pick your product (perfume, power bank, laptop, dry ice, nail polish, paint, perishable food…), answer a few questions about quantity, packaging and carrier, and get USPS, UPS and FedEx side by side — ship / conditional / don't buy the label — with the exact requirements and a cited official source plus verification date on every row.
It also refuses to guess. If a critical fact is unknown, you get MOREINFONEEDED or CONDITIONAL with a list of exactly which facts are missing — never a confident "probably fine". For the USPS fee specifically there's a path that just says don't buy this label before you're at the counter.
It's 2026, so the obvious question is why this isn't a chatbot. Three reasons: Determinism is the product. The verdicts come from a rule engine running over versioned JSON rule sets. Every rule row traces to a source ID with a verification date and a content hash, and the rule directory ships with a validation suite that rejects duplicate product IDs, conflicting aliases, and sources from unofficial domains. If someone asks "why did it say don't ship this?", the answer is a traceable chain, not a plausible paragraph. LLMs are on the input side, locally. The AI-adjacent features — parsing text you paste from an SDS, reading battery label photos for voltage/Ah/Wh hints — run entirely in your browser. They produce evidence cards with extracted values, source section, and confidence. You confirm them; only then do confirmed facts feed the deterministic engine. Photos never leave the device. Privacy is a feature, not a promise. No account. No emails. Server-side analytics are anonymous funnel events without IPs, raw URLs, or your answers. A SDS PDF you read stays local.
The same rule runtime also powers a stateless API (POST /api/v1/check — you send confirmed product facts, it returns the three-carrier result plus a decision trace with rule versions) and an MCP tool, so a script or agent can pre-check a cart before checkout.
It's a solo project, early. ~25 product risk profiles, US domestic, three carriers. Traffic is tiny but converting well — the people who land on it do finish checks. The gaps I know about: no batch catalog checks, no multi-item same-box compatibility, and the coverage is nowhere near "every SKU".
Feedback I'd genuinely value If you ship batteries or flammables regularly: do the outcomes match what you're told at the counter? Disagreements are exactly what the correction form is for. If you've built anything with a "deterministic core, fuzzy shell" architecture: how do you keep the fuzzy side from leaking into the core?