I built a userscript and Firefox add‑on that checks if EU discounts are real

Description and use

truediscount is a browser extension that calculates the true discount based on the EU 30‑day lowest price rule, providing transparent savings information to shoppers.

Problem

Online retailers often display large discount badges while the actual discount relative to the 30‑day lowest price is much smaller or even negative, violating EU Omnibus Directive rules.

Outcome

truediscount provides shoppers with accurate discount calculations based on EU regulations, improves transparency, and operates entirely locally without a backend.

Workflow

• 1. Load a product page on a supported shop.
• 2. truediscount extracts the current price, the advertised "was" price, and the 30‑day lowest price using locale‑aware parsing and language‑specific phrases.
• 3. It recalculates the discount as (30‑day lowest – current)/30‑day lowest and displays the result next to the shop’s badge.
• 4. It stores price data locally in the browser, building a graph of seen prices.
• 5. Users can install the tool via a userscript, Firefox add‑on, or Chrome‑based unpacked extension.

Evidence

Since 2022 the EU Omnibus Directive (Article 6a of the Price Indication Directive) says that when a shop announces a price reduction, the "before" price must be the lowest price it charged in the previous 30 days.

Source

truediscount finds the current price, the advertised "was" price and the 30‑day lowest price on a product page and recalculates the discount against the 30‑day lowest price and shows it next to the shop's claim.

Source

Price parsing is the hard part. "1.299,00 €", "€1,299.00" and "1 299,00 zł" all need to end up as the same number.

Source