Should this Offline Marketing trick be Legal?

Today I received an envelope in the e-mail. It was addressed to me and it was rather thick. I had no clue what it was, but as soon as I opened it I realized that it was an interesting marketing trick. Actually, it was a rather sleazy marketing trick. The envelope was from an institute of foot and mouth painters (people that can`t use their hands), and it contained many envelopes and christmas postcards that their members had painted. And finally I found an invoice for about $10 with an interesting text. This was all in Norwegian, but it said something like "you can pay for the cards or you can return them if you want to, but you don`t have to do either of this." What they did was send me the product, tell me that I can pay for it or not. They didn`t say that if I like the cards, I should pay, but that`s what I thought when I looked at all the nice cards. How can I keep all those cards without paying for them? I was unsure what to do, but then it hit me that this was just a marketing trick. I don`t have a problem with it really, but I would guess that a lot of people might have. Because this is all about psychology and that you might be feeling some kind of guilt if you don`t pay for the product. And when the product is being made by people with a disability, you will probably feel more guilt if you don`t pay for the product. What do you think?