dryer duct extension/transiition

perhaps this post should be in be discussing the retailer (costco), but anyway i ordered a dryer from costco which offered white glove delivery (ie they install it for you)

so the guy said he cant install it because im using a dryer duct extension (dont most people use them?) ...said something about a liability issue (i mean wouldnt all installations?) ,...just curious if this is a valid excuse or should i call costco and insist they have them install it?