"I'm willing to keep it but just wanted to let you know."
Yeah that sort of email says you lot gave me the big hose and I'm keeping it so blurrr
What you should have written is:
"Hi, I just received the Python gravel cleaner. I ordered the 25 foot version but got the 50 foot version. Do you want me to send it back so you can send me the 25 ft one?"
Then you would either get a response of yes ok, here is a shipment number. Just put it on the item and send it back. Or, Thank you for informing us of the error. Please keep the 50 foot model as a gesture of good will.