mumsbestfriend,
sometimes it is better to LOOK silly then TO BE a fool later.
If you ask them - nobody will eat you.
you have the right to ask and to have an answer. If they are polite and careful - they will explain you well.
If they don't to - just forget and say "happy me that I haven't sent them my money".
I don't know what they mean when speaking about additional 150$, but you have a chance to discuss it with this Chinese Company.
If you want - tell us what they told you.
Don't shy - this is a key factor.