Did you still have this activation code via email? Or did you pull some kind of log of your activation keys? If the latter, chances are you pulled an incorrect key.
If the former, I have no clue. But if MS are telling you a key starting "00" is not valid.... it's not valid.
Again, this is an issue for SoftwareAU.... who may or may not be legit/authorized. Lots of claims of enterprise keys being sold by this company.
To quote one claim I've found via a 2 second Google search:
I purchased Msft Office 2010 & received a Product Key. Everything worked fine for 2 years when I had to reinstall the app after a system failure. The key was no longer valid. Msft said that it was an enterprise key & is blacklisted because it had been provided to other customers. I notified softwareAU of the situation & have received no response. It appears that the software you will receive will activate with the product key provided, but once softwareAU exceeds the limit for the distribution of the key, the key is then blacklisted as pirated. Don't buy from this company!
Sounds a little familiar.
FWIW, Office 2010 (Home & Student) was available as a "family pack", where you could install on 3 systems.
The individual cost was $149 MSRP..... I assume the "family pack" had a break in pricing but, even assuming it didn't....
3x $149 = $447
https
/arstechnica.com/information-technology/2010/01/microsoft-reveals-office-2010-retail-prices/
If you bought this in 2014, 2010 had already been replaced (by Office 2013) so would've been either discontinued for sale, or heavily discounted by MS - I'd be relatively confident that an authorized MS reseller would've been selling Office 2013 in 2014, not 2010.
Ultimately, the price paid is irrelevant. Whether it's $2 or $200 doesn't actually matter here. IF bought from an an unscrupulous outlet, selling a volume licensing key that has since been blacklisted (which, based on information I can find, sounds like the case),.... that money, for all intents & purposes, is gone.
You could try talking to MS again. Perhaps they'd take pity on you and offer a discount on Office 2016? No guarantees, but wouldn't hurt to ask. A shouting match won't work in your favor though, but a more reasonable approach might.