I looked at the link for a while...it doesn't appear to be one of the typical 'assembly' scams...since you will be the one selling the items to others...
I think you are correct, they are only charging the fee for cost of materials and instructions....
Occasionally, I will see offers similar to this, where the website is charging three or four times the actual cost (to them) for the 'kit' contents...I would definitely recommend checking around for prices on the metal hardware, webbing, etc...I don't know what the prices for these items should be, just make sure the prices aren't exessively high compared to other hardware suppliers....
The fabric collars, etc are very attractive...but I also know from doing crafts of my own, the prices on yards of those nice fabrics can be astronomical...not trying to discourage you--just saying there may be more cost involved in making an item than you might think at first...I'm pointing this out because the website says these are made with "your favorite fabrics"...and the fabric is not included in the kit list...meaning you will have to buy it out of pocket...