NOT A TEACHER
Thank you for your kind note.
As the other posters have told you, what is "right" is open to discussion.
Of course, I cannot tell you what is "right."
I can, however, tell you that one reputable grammar advice website says that a period is needed in:
"Could you please send me your catalogue
." That website (
englishclub.com) claims that it is a polite request
in the form of a question, so it takes a period.
Of course, other experts would tell you that it should (or "can") take a question mark.
I think that a question mark could be considered more polite. So it could not hurt to use a question mark in
your example. On the other hand, some people might think that a question mark in your example might show that you
are "begging" while a period might indicate a polite but firm request.
Furthermore, consider the cultural aspects. In some cultures, people are very careful to show great respect and
deference to "superiors." A question mark or period might make a big difference to some people who read the sentence.
If you do not use a question mark, that might indicate that you are
expecting the documents by next Monday. That
"order" might possibly upset the person who reads your email.
Let's put it this way: you cannot lose anything by using a question mark. But it is possible that using a period could
upset someone.
I suggest that you go with your gut feeling. In other words, do what you think is right. And, of course, accept the
consequences.
Best of luck to you!
P.S. Personally, I would use a question mark to my provider. I am a very timid soul who is very deferential to my superiors. I would not feel comfortable giving an "order" to my provider. If they did not provide the documents by next Monday, then I would be left in a very embarrassing situation. My "order" had been disobeyed.
In the case of the "catalogue" sentence, I definitely would use a period, for there is no time limit. It's really just a nice
way to say ""Please send me a catalogue." (If they do, they do; if they don't, they don't. I don't care that much.)