Be sure the preview your gerber files before submitting. Download a gerber viewer(gerber, ViewMate), install, and take a look at your own gerber files. If they don’t look right, or layers do not line up, or if they don’t import at all, stop, fix your output settings and open the gerbers again.
Are there any custom duty and tax for samples and small order?
It depends on the where you are. If you need we can make low invoice value for custom declarations.
