Flutter POS Estimate & Discount Fix

I have a Flutter-based point-of-sale app that already handles sales, returns and daily reports. Two gaps remain:

First, the system must be able to create an estimate (quotation) straight from the cart screen and let the user export or print it. I am open to the final file format—PDF, Excel or Word—as long as the layout matches the existing receipt style and can be saved or shared from both Android and iOS builds.

Second, the rerun module is producing wrong figures whenever a discount is applied. Core sales totals and taxes are fine; only the discount logic misfires, so totals after discount do not reconcile with the day-end report. All calculation code sits in a single Dart service class, and the discount rules are simple percentage or fixed-amount values pulled from Firestore.

Deliverables
• Add an “Generate Estimate” action on the cart/checkout page that outputs a well-formatted document and stores a copy in the local device and Firestore.
• Refactor the discount calculation in the rerun module so totals are always correct and match transaction history.
• Push clean, commented code to our Git repo and supply a short change log so QA can verify.

Acceptance criteria
1. Creating an estimate reproduces cart items, taxes and discounts exactly and opens a share dialog within three seconds.
2. Applying any discount type shows the same net amount in the rerun module, the sales summary, and the printed receipt.
3. All unit tests pass and new tests cover the corrected discount logic.

The project is small in scope but mission-critical for daily operations, so clear commits and the ability to walk me through the changes on a brief call will be appreciated.