What if you pay $1 for the trip?
What if it's 50% off the trip?
...
Is a social referral program an "ad"?
There's plenty of gray area where you're gifting something non-cash, like an experience that you are interested in anyway. Look at the Kylie Airbnb deal for instance.
There's not. Journalism and politics have this covered:
NO GIFTS.
It's really that simple.
Basically, if there is even a remote chance that the action could be misconstrued as a gift when viewed from the outside, it's politely rejected.
One time this created a funny situation where a pro sports team flew us out on their private jet. While we were in the air, they were about to hand out hats with their logo on them, and the head of our group had to stop that until he could consult the lawyers about that gift. It felt like an overreaction in the particular situation, to say the least.
Uh, every single time a "journalist" is easy on a subject so that they can get future access, that's a gift from the subject in exchange for them going easy.
Journalism as actually practiced is almost a definition of what a "gift" culture is, especially political journalism.