Improve currency amount description

[skalexch]

Summary

This documentation change elaborates on the description of the "amount" field in fare_products.txt.

Describe the Problem

There is a recurring practice of GTFS feeds having invalid currency amounts, e.g. If the fare is 2USD, the value of the field is specified as "2", instead of the norm-compliant "2.00".

The spec mentions the ISO 4217 norm in the field type description. However, if is beneficial to mention it again in the field description and to add an example.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Proposed Discussion Period

This is a documentation change. We will keep it up for a few days to get reviews, and then merge.

Proposal Update Tracker

Date	Update Description
(2026-02-23)	(Opened PR)

Checklist

• I have read and understood the GTFS Change process.
• I have read and understood the GTFS Guiding Principles.
• I understand the Role of Advocate and the Responsibilities attached to it.
• I have signed the Contributor License Agreement (CLA).
• I have ensured that this proposal is not covered by an existing Pull Request.

[Sergiodero]

Thanks @skalexch, from my end the proposed text looks good and aligns well with the current content.