You're helping a small crafts seller who ships handmade goods and constantly forgets how many stamps a lumpy envelope needs. Build a single-page calculator where a user enters a package's weight (in ounces), selects the mail type (First-Class Letter, First-Class Large Envelope, or Priority Mail Flat Rate), and instantly sees the estimated postage cost based on fixed, hardcoded USPS-style rate tiers (no live API needed — just bake in the tier table).

The UI should display a clear cost breakdown: base rate, any weight-overage fee, and the total. Include a small history list that shows the last 5 calculations (weight → mail type → cost) so the seller can compare options without re-entering values. A 'Clear History' button resets the list. All state lives in memory; no backend required.