Again, classical Latin does not use the subjunctive in this way. Visne should be the correct verb here. Independent uses of the subjunctive do not show politeness.
Interearing! I was wondering how good the course was, apparently there are some rough edges
how should one use it then? Could you give an example with velisne from this sentence?
If you were to give me olives as a present, you would ask me "velisne olivas?"
How to say "Would you like a (single) olive"? Olivam?