I'm putting together a site that will sell magazine subscriptions for our in-house published periodicals. We have one that people can purchase at a discount if they take out another subscription.

What I'd like is a dropdown that people can pick from before they click "Add to Cart"

Magazine A - UK Subscription (£30.00)

Would you like to add Magazine B for £5?
> Yes
> No

If they select "Yes", the option is added, and their total for this item becomes £35.00

How, without creating additional products in the system, would I do this?

Actually, if I have to create additional products to represent the combinations of with and without B, that's not a lot of work, I just figured there'd be an easier way!