^ Now you have a different story, because now someone you mentioned it to, is telling you there is no reason to replace the whole CV axle at a premium price instead of just the boot, unless there was so much grease loss, or enough grime got in, to tear it up. If it is not excessively worn, what is the rationale to pay hundreds of dollars more? That might be what a shop tells you, because it's not their money and they would rather not refurbish anything, even a paperclip with a stray bend in it (lol!).
This is within the context that aftermarket CV axles are usually junk, there there typically isn't any sub-$250 OEM replacement option. Varies by vehicle.
Stretch boot though? I have never used one, would use a normal boot with straps. Do they spec what kind of rubber it is?
Whether you can take that shortcut (besides if it is a durable product) depends on how much grease was lost or how much grime intrusion. The more conservative answer is you want to take it apart, clean it out and pack with fresh grease. Some people have a mental block about cleaning things, but a 5 gallon bucket and a half gallon gasoline (plus rinse, and rubber gloves) should do fine.