Adding a sub panel in my basement
Take the subpanel back and get another one, with an eye toward two features: 


Auxiliary ground bus included, not an extra-cost add-on
lots and lots of spaces.  Right now in your construction, spaces are dirt cheap; regrets are expensive.  Especially if drywall is involved. 


Also look for 


being bundled with useful "bonus" breakers
being a quality type (Siemens, QO, GE Qline, CH)
having sensibly priced AFCIs ($40ish) and GFCI/AFCI combos ($55ish). Can't use brand X breakers on brand Y panels; that's not brand loyalty but a matter of proper fit. 
same type as your main panel (if you care)


It does not matter what the panel ampere rating is, as long as it is >80A because that is the breaker to use with your 75A 2/2/2/4 cable.*  There is nothing wrong with getting a 200A subpanel for instance, as long as it has a plentiful supply of breaker spaces.  Headroom increases safety and also allows for easy upgrades (just replace the cable/breaker).

The subpanel does not need a main breaker, but if it's cheaper (with bonus breakers) to accept a panel with one, that's fine. 



* 75A -- yeah, the advice you got was that wrong.  Never take advice from big-box again. In fact let's get you shopping at a proper electrical supply house.  Most are locally owned businesses, and most will be happy to deal with you at sensible prices.  Be honest with them that you're sick of dealing with big-box and their limited selection**, terrible advice and you're heard electrical supply prices are actually better on most things.  

** I suspect that's why they put you into 2/2/2/4 Al.