No delivery despite full payment in advance

Ordered a treadmill several weeks ago, paid full amount in advance plus delivery fee. Got a demand from their selected courier for significant additional charges (supposedly refundable). I refused to pay more with the result that the item has not been delivered and I am out of pocket. Curious lack of response from either MT Gym or the courier makes me wonder about their legitimacy. I have full contact details of both so, if the businesses actually exist, I will get to the bottom of it. If anyone else has had a similar experience, let me know if you would like to join forces.