I think you are right, Rodolfo. The writer should have said "If you have any questions,.." or "If you have any queries..". I suspect that the writer was a native Portuguese or Spanish speaker, and that they were influenced by their first language. "Doubts" sounds like the wrong word to use in that context. In fact, if I were in your position, I would have some serious DOUBTS ( apreensões) about this language course. If you have doubts about something, this means that you're suspicious about it. If there are English language mistakes in the promotional material, I really wouldn't trust this course. In this case, you really should have 'doubts'..... but that isn't what the writer had in mind!

It's not wrong because the doubts could relate to the accuracy of the information in the letter. But it sounds unnecessarily negative and it would be more natural to use "queries" or "questions", in my opinion.

I agree with the other comments Doubts is very negative and a little strange