$74.99 is the best price I've seen for these in a while. Even with tax it's about the same or slightly better than the best price at the moment, and then you also get the generous Costco return policy.
These items don't show up online, but it seems it's a new item that only some Costco stores are carrying. If you go to the New in the Warehouse section of costco.com (see link below) and check the locations near you, some may have it listed. For me, it shows up in the Major Appliances section, with costco item number 558599. I tried calling a costco that didn't have it listed, but they didn't carry it and the only stores they said that had it were the same ones that listed it on the website (so it's probably a safe assumption that if it isn't listed for a particular warehouse, they probably don't have it)
The major difference between DOCSIS 3.0 and DOCSIS 2.0 modem is that 3.0 supports multi channeling. Motorola 6120 supports 4 downstream and 4 upstream channels. That means if there's alot of cable modems running on the same node (like in many urban cities), DOCSIS 2.0 modem performance will suffer because they all share the same channel while DOCSIS 3.0 modems will not suffer as much because it supports multichannel.
How can you test if you might need a DOCSIS 3.0 modem is that you go to speedtest.net and check your network speed at a non-busy time, like 8am in the morning, and compare it to prime time like 8pm. If you are not getting your advertised speed at 8pm and you know that your ISP rolls out DOCSIS 3.0 modems to other customers, DOCSIS 3.0 modem will mostly likely help. If you are getting your advertised speeds at 8pm, getting an DOCSIS 3.0 modem is a waste of money. The only other reason why you want an DOCSIS 3.0 modem is if you are getting an internet service that is more than 45mbps.
PM from Amazon $79.99 to Staples and use a $25 off $75 EJ coupon. Total should be about $60 after tax and cost of coupon. Sounds like the PM is already in the Staples system so the PM should be easy.