Buy x from collection 1 get discount on item from collection 2.
Example: buy 3 bottles of product get a travel bag 25% off.
Assuming there are multiple travel bags so there is a travel bag collection and bottled products are in their own collection. Fake example but hopefully it gets the idea across.
I am always running into brick walls trying to increase add-on sales with wix and this is one of many things that would help.