Bulky and heavy items are not well suited to mail order. But all your competitors are in the same boat.
Who do you think will be buying from you online? That may give you some clues about what the market can bear.
Also phone some of those freight companies and have a discussion of your needs and expectations. You are fortunate since you are working for an established business and won't have to prove your bona fides.
One off shipping is usually more expensive than annual contracts, btw.
At one time I was selling cast iron wood stoves and sheet metal wood burning furnaces and shipping across Canada. The cost of shipping is part of the cost. It is what it is and your customers, particularly those in rural and remote communities are already aware of that.
You should also experiment with Calculated Shipping on the Sell Your Item form. I believe US sellers have a couple of companies that can be chosen from there. Fool around with it. See what it looks like to ship a heat exchanger or solar panel to Alaska or Maine or Guam. Think about the product line you will be listing here.
