From experience with using carriers tracking information world-wide: scraping will be fine for low volume usage and if we are talking personal use cases.
If this has any professional smell (as in you make money with whatever you use the data for) you will want and need access to each carriers API for several reasons:
- accuracy of data: this is not guaranteed for the web views, they may (and are) usually outdated by hours - volume of updates: carrier APIs usually come with rate limitations and can also require contracts for uses
Why scrape when, at least in the US, all the major players (USPS, UPS, FedEx) have free API that you can use to query tracking information?
Which region/country are you referring to? What's the use case?
IANAL, but I used to work at one of the large transportation providers. For the high-volume tracking use cases, web site scraping probably won't cut it (too slow, probably get blocked by the carrier). Most high-volume trackers used the carriers' APIs. It was sort of a love-hate relationship and my carrier would sometimes block high-volume carriers who ran refund services on behalf of other customers.
In short, whether it's "legal" or not, if you are doing a high volume of tracking for refunds you will be at the mercy of the carrier who could cut you off at anytime. A firewall against that is to have a bunch of high-volume paying customers of that carrier who would pitch a fit if your service was turned off.
[Edited for clarification]