Automatically add a product to cart?

Automatically add a product to cart?

Hi is it possible to conditionally add an item to the cart automatically if a particular product has already been added?

i.e. to be more specific I am creating a shop through which members can buy membership and event entries.

If they enter an event I want to automatically add a first aid contribution. BUt only once; i.e. if they enter multiple events they need only contribute once.

Any suggestions most welcome!

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