Currently there is no Amazon rule against this, and I don’t think legally Amazon can make it as such.
However, the Amazon system does not currently handle different prices as you can see from the screenshot.
So to achieve this you are either going to have to create two sets of inventory with seperate SKU’s and prices.
The downside of course is you will be unable to manage your stock control if you do this from the Amazon platform.
Or alternatively you could simply run a promotion on the webstore to reduce the cost of the item or even consider reducing the shipping cost on your Amazon webstore products.