Yup, you have to pay using Zip. The cost for me was $6 total interest fees on a roughly $1000 purchase (of unrelated items, but all sold and shipped by Newegg), on which the promo code zip111 gave me 11% off. So you save $111 but pay $6 in interest fees. You have to use a cc to make the zip payments, one of which is immediate and three come later at 2-week intervals. My assumption is that Zip is funding this promo in order to grow their customer base. Unfortunately for them, after I've made all my payments (in 6 weeks), I plan to simply close my Zip account.
BTW, I never got a response from Microsoft as to the 15% Cashback I should have received for my initial purchase of the Seagate 20TB drives on Newegg. At the initial price of $280 each, I felt that was too high without the 15% rebate. So I simply returned the drives to Newegg, full refund (I had not opened any of their sealed packaging).
In their place I purchased six WD 18TB Ultrastar DC HC550 SATA drives, factory recertified, from Server Part Deals, for $200 each. I'll post warranty info and SMART data after I receive them in a couple of days.