The LL Administration has a policy of selling 3rd party services such as travel and clothing. These "Travel" letters and other ads are sent to you monthly or more frequently. You cannot block these and LL tells you in so many words "deal with it" and "delete them yourself".
We have asked the Admin to review their policy since any decent business would let their customer decide what they would like to receive in terms of advertising.
There are plenty of space already, including the front page of LL that advertises these services.
If this policy bugs you too, we encourage you to tell admin, we're pretty sure we're not the only ones.