Track 35 USPS packages in one API request.

curl https://api.shipmenttrackingapi.com/v1/track \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "carrier": "usps",
    "trackingNumbers": [
      "YOUR_TRACKING_NUMBER_1",
      "YOUR_TRACKING_NUMBER_2",
      "YOUR_TRACKING_NUMBER_3"
    ]
  }'

const res = await fetch("https://api.shipmenttrackingapi.com/v1/track", {
  method: "POST",
  headers: {
    Authorization: "Bearer YOUR_API_KEY",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    carrier: "usps",
    trackingNumbers: [
      "YOUR_TRACKING_NUMBER_1",
      "YOUR_TRACKING_NUMBER_2",
      "YOUR_TRACKING_NUMBER_3",
    ],
  }),
});

const body = await res.json();
if (!body.success) throw new Error(body.error.code);
for (const r of body.data.results) {
  console.log(r.trackingNumber, r.status ?? r.error.code);
}

import requests

res = requests.post(
    "https://api.shipmenttrackingapi.com/v1/track",
    headers={"Authorization": "Bearer YOUR_API_KEY"},
    json={
        "carrier": "usps",
        "trackingNumbers": [
            "YOUR_TRACKING_NUMBER_1",
            "YOUR_TRACKING_NUMBER_2",
            "YOUR_TRACKING_NUMBER_3",
        ],
    },
    timeout=60,
)

body = res.json()
if not body["success"]:
    raise SystemExit(body["error"]["code"])
for r in body["data"]["results"]:
    print(r["trackingNumber"], r.get("status") or r["error"]["code"])

// Node 18 or later, saved as track.mjs
const API = "https://api.shipmenttrackingapi.com/v1";
const headers = {
  Authorization: "Bearer YOUR_API_KEY",
  "Content-Type": "application/json",
};
const sleep = (ms) => new Promise((r) => setTimeout(r, ms));

// Your USPS numbers. Remove spaces and repeats: the API rejects both.
const list = ["YOUR_TRACKING_NUMBER_1", "YOUR_TRACKING_NUMBER_2"];
const clean = list.map((n) => n.replace(/[^0-9A-Za-z]/g, "").toUpperCase());
const numbers = [...new Set(clean)];

// How many numbers one USPS request takes. This call needs no key.
const { data } = await (await fetch(`${API}/carriers`)).json();
const size = data.carriers.find((c) => c.id === "usps").maxTrackingNumbers;

const results = [];
for (let i = 0; i < numbers.length; i += size) {
  const group = numbers.slice(i, i + size);
  let res;
  for (let attempt = 1; attempt <= 3; attempt++) {
    res = await fetch(`${API}/track`, {
      method: "POST",
      headers,
      body: JSON.stringify({ carrier: "usps", trackingNumbers: group }),
      signal: AbortSignal.timeout(60_000),
    });
    if (res.status !== 503) break; // a 503 was not counted
    await sleep(5_000);
  }
  const body = await res.json();
  if (!body.success) throw new Error(body.error.code);
  results.push(...body.data.results);
}

# Python 3, with the requests package
import re
import time
import requests

API = "https://api.shipmenttrackingapi.com/v1"
HEADERS = {"Authorization": "Bearer YOUR_API_KEY"}

# Your USPS numbers. Remove spaces and repeats: the API rejects both.
raw = ["YOUR_TRACKING_NUMBER_1", "YOUR_TRACKING_NUMBER_2"]
clean = [re.sub(r"[^0-9A-Za-z]", "", n).upper() for n in raw]
numbers = list(dict.fromkeys(clean))

# How many numbers one USPS request takes. This call needs no key.
carriers = requests.get(f"{API}/carriers", timeout=30).json()["data"]["carriers"]
size = next(c for c in carriers if c["id"] == "usps")["maxTrackingNumbers"]

results = []
for i in range(0, len(numbers), size):
    group = numbers[i:i + size]
    for attempt in range(3):
        res = requests.post(
            f"{API}/track",
            headers=HEADERS,
            json={"carrier": "usps", "trackingNumbers": group},
            timeout=60,
        )
        if res.status_code != 503:  # a 503 was not counted
            break
        time.sleep(5)
    body = res.json()
    if not body["success"]:
        raise SystemExit(body["error"]["code"])
    results.extend(body["data"]["results"])

curl https://api.shipmenttrackingapi.com/v1/carriers