Creating a discount that supersedes another discount

I would like to create a discount for my users that will supersede a discount they receive when they are in session.

Currently, my users (members) are broken up into userLevels 1, 2 and 3. Each userLevel receives a discount based on their Membership level. For example, userLevel 1 gets a 25% discount, userLevel 2 gets a 20%, and so on.

My company is looking to market webinars for a modest price. However, I would like to extend our webinars to our members at no cost.

The best way I can think of, at the moment, is to create a member discount based on a session variable value (e.g., userLevels) and calculate the discount at 100% - thus charging the member nothing. For non-members or guests, they would have to pay the fee.

However, I am uncertain how to put together the discount. I am concerned that since our members already receive a discount off when they are logged in, that the 100% discount will be added onto their current discount or cancel the discounts altogether. I have never done anything like this before.

Ideally, I would like a discount that takes 100% off the cost of a webinar, and just the webinar, when a member is logged in. For example, if a member is logged and enrolls in a webinar and also purchases an unrelated product off our website, I would like eCart to calculate the discount for the webinar separately from the unrelated product.

Lastly, I have created a "discountable" column in my webinars database so that eCart and differentiate between the webinar from any other products we currently sell.

I have also attached a copy of files for your review. Thank you.