All fansites give two ways to get Theodore Loveless' key. I will quote these two ways here from Tibia Wiki.
„First method: Go to Liberty Bay. Go the Jewel Shop, north east corner of Liberty Bay. Talk to Chantalle and ask her for "key". She says it is not possible to bribe her, she also says that Herbert, the postman in Liberty Bay, is easy to bribe. Now go to Herbert (inside the depot) and say "hi", "letter", "gold", "yes". He will take 1,000 gps. from you in exchange for a letter. Now, head Venore and find NPC Carina (bank, east depot). She will sell you a Precious Necklace for 5,000 gps. Once you have the precious necklace, go back to Liberty Bay and talk to Chantalle again. Say "Key", and she will give you the key.
Second method: You wear a Post Officers Hat in your helmet slot and talk to Bertram if your character is female or Chantalle if your character is male to get the key and complete this mission. (Note: You must be the Arch Postman rank for this to work!). Ask about a key and he/she will mention how handsome you look in your uniform and then give you the key for the door upstairs.”
I've been doing some research on this quest and discovered another method for obtaining the key. After so many years since the quest was introduced into the game, this was a really surprising and intriguing discovery for me. But let me start at the beginning. I'll explain how I came up with this new method. One time I was talking to NPC Pig. She told me her name is Shantalla.
This name is very similar to Chantalle which is another NPC living in Liberty Bay. So I went to talk to Chantalle. As we know the NPC Pig is involved in the quest "Kissing a Pig". Therefore, I asked Chantalle about the kiss. Look at what Chantalle responded to the word "kiss."
Her statement clearly refers to a pig. I figured it couldn't be a coincidence and there was some mystery to investigate. I also noticed that Bertram, the NPC standing near Chantalle, also reacts to the word "kiss" in the same way.
Both Bertram and Chantalle are involved in The Thieves Guild quest. So I decided on another character to fully complete the "Kissing a pig quest" to learn the art of kissing, and try to complete the 6th mission of The Thieves Guild Quest in a different way than the one given in the spoilers.
I went to Chantalle and demanded a key.
She wouldn't give me the key so I asked her about the kiss and it happened:
I kissed Chantalle and she then gave me the key. Oh! She is really hot! I tried kissing her again but failed
11:50 Norghin : hi
11:50 Chantalle: Hello. What brings you here?
11:52 Norghin : key
11:52 Chantalle: Are you insane?! I'd never misuse the trust placed upon me and hand you Master Loveless' key! I'm not as bribeable as Herbert, this bad example of a postman - despite his handsome uniform.
11:52 Norghin : kiss
11:52 Chantalle: Oh, I've heard you are the king of kisses, is that true???
11:52 Norghin : yes
11:52 Chantalle: Hmmm. If it's true, I'd give you the key you are looking for in exchange for a kiss! Are you interested?
11:53 Norghin : yes
11:53 Chantalle: Ohhhh! Th .. Thank you! Here is .. your key!
11:53 Norghin : kiss
11:53 Chantalle: No, thank you. I fear I might get addicted <sighs>.
So another way to get the key is to fully complete the "Kissing a pig quest" and ask Bertram or Chantalle for a kiss. According to my research, this method has never been published on any Tibian fansite and remained unknown to the community until now. But the discovery of this "new" method raises new question. Perhaps the key can be obtained in yet another way? That's why I ask the question - how many methods are there to obtain Theodore Loveless key?