Quandary

This sort of thing is situational.  If you have been a good customer and they have reason to expect that you would make a purchase if you like it, this would seem like a too high charge.  If the fee were refundable if you make the purchase that is another story.  It might take two people to deliver and set up the amp and they would have to give you a reasonable amount of time to audition.  Their cost might actually be close to this charge.  You might negotiate a lower charge and explain that the lower charge means both you and the store have some skin in the game.

Most shops that do offer home demonstration will do so only with customers they are familiar and can trust.  They will help you load it in your car, but beyond that, it would be a big imposition to expect them to set up in your home on the hope of a sale.  The crediting to your purchase price the fee for the demonstration is what I expected.  The question is whether $300 is reasonable.  If they give you enough time to seriously audition the amp, I think it is reasonable.  Ideally, they would leave it for a few days and return to pick it up.  
 Pass makes some very nice sounding amps, and the smaller amp you are considering will probably sound very similar to the higher powered amp if your speakers are efficient and the two amps have similar topology and are comparable in quality level.  I’ve even heard comparisons where the lower-powered amp by a particular manufacturer sounded better than its more powerful sibling.