Please could you release orders for March to June 2019.
Using a question mark would change the semantic meaning.
I've heard people say this form both with the raised ending indicating a question and without. I'm less confident than Kris that the ordering between "Please" and "could" is semantically significant, but this is absolutely a candidate for the "politer than thou" do this now! form.
I can imagine some contexts that could potentially suggest it was more likely to be an actual question - specifically, if the person asking doesn't have the authority to give such an order. But I expect that the context is that the person making the statement has the authority to direct the person who would release the orders to release them, but does not have the access to do it themselves. It's also possible they do have the access, but are wanting to exercise their authority over the person they're directing.
Yes. "Could" marks it as a question, so it needs the appropriate punctuation mark.