2007 Forester rear struts

O.k. Here's what's happening. Left rear strut rusted out bottom spring retainer and is now rubbing against rear tire and wearing groove in tread. Got quote from dealer to replace both rears at $875! Ouch! I priced out some KYB OE brands at less than $110 each from local parts store. Not only that but dealer says need an o.k. to do it (and parts payment of course) so they can order parts---they said aftermarket struts not Subaru part. Is $400+ in labor a little ridiculous? I'm no spring chicken and thought about doing it myself but my wife thinks it's too big a job for me to handle (I'm 67 on fixed income). Is it really that difficult to do, using a strut spring compressor of course?