How to add a fee for all products of a certain category? I.e. Deposit Fee.

Hi,

So is that something your customers have to pay on top of the price of the product ? Or something you want them to pay instead of the price of the product ?
And if two such products are in the cart, is the deposit added for each product or only once for the whole cart ?

Hi,

You have to configure your tax rules as necessary, follow me step by step :
1. You first need to configure these two tax rates via the "manage tax rates" button of the System => Taxes menu.
2. Create a new tax category via the "manage tax category" button there and then assign this product to that tax category so that it gets these tax rates and not the ones you already have for the other products.
3. Create two tax rules via the menu System => Taxes, one for each tax rate, with that tax category selected and the "cumulative tax" setting activated for both.

Results, both tax rates will be added to that product price(s) and displayed like in your screenshot.
Hope this will fit your needs.

Regards